Update to return correct value for multiple_domains().

requests/cookies.py

Adjust flow to add domains to list as the first operation in the loop, then convert to a set and check the length of that set is greater than 1.

Add tests to pass for a single domain, two cookies with the same domain, and multiple domains.

Perhaps not the most efficient method, but it works. 

#4557 

@nateprewitt Thank you kindly for the helpful feedback!

I've adjusted the method to make use of set in place of the domains list.  I've also adjusted the structure of the conditional a bit and added an additional test check that null values are handled correctly by the method.

@nateprewitt You're very welcome, sir.

The other updates you requested have been made.

@gahancorpcfo, It looks like you may have accidentally merged master into your commit. Would you mind only including the changes related to this PR. The easiest way to do this is likely to reset HEAD~1 and then `git add -p your-relevant-files` or cherry pick from another branch if you have it.

I believe I've managed to produce a single commit with the relevant changes that was not merged with master. 

. . . I hope. 

@nateprewitt Thank you very much for your help with this. 

# [Codecov](https://codecov.io/gh/requests/requests/pull/4581?src=pr&el=h1) Report
> Merging [#4581](https://codecov.io/gh/requests/requests/pull/4581?src=pr&el=desc) into [proposed/3.0.0](https://codecov.io/gh/requests/requests/commit/9c6bd54b44c0b05c6907522e8d9998a87b69c1cd?src=pr&el=desc) will **increase** coverage by `0.06%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/requests/requests/pull/4581/graphs/tree.svg?width=650&height=150&src=pr&token=mZGswIiXLa)](https://codecov.io/gh/requests/requests/pull/4581?src=pr&el=tree)

```diff
@@                Coverage Diff                 @@
##           proposed/3.0.0    #4581      +/-   ##
==================================================
+ Coverage           90.51%   90.57%   +0.06%     
==================================================
  Files                  14       15       +1     
  Lines                1549     1549              
==================================================
+ Hits                 1402     1403       +1     
+ Misses                147      146       -1
```


| [Impacted Files](https://codecov.io/gh/requests/requests/pull/4581?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [requests/models.py](https://codecov.io/gh/requests/requests/pull/4581/diff?src=pr&el=tree#diff-cmVxdWVzdHMvbW9kZWxzLnB5) | `93.67% <ø> (-0.9%)` | :arrow_down: |
| [requests/status\_codes.py](https://codecov.io/gh/requests/requests/pull/4581/diff?src=pr&el=tree#diff-cmVxdWVzdHMvc3RhdHVzX2NvZGVzLnB5) | `100% <100%> (ø)` | :arrow_up: |
| [requests/sessions.py](https://codecov.io/gh/requests/requests/pull/4581/diff?src=pr&el=tree#diff-cmVxdWVzdHMvc2Vzc2lvbnMucHk=) | `95.63% <100%> (+0.03%)` | :arrow_up: |
| [requests/help.py](https://codecov.io/gh/requests/requests/pull/4581/diff?src=pr&el=tree#diff-cmVxdWVzdHMvaGVscC5weQ==) | `68.62% <100%> (ø)` | :arrow_up: |
| [requests/cookies.py](https://codecov.io/gh/requests/requests/pull/4581/diff?src=pr&el=tree#diff-cmVxdWVzdHMvY29va2llcy5weQ==) | `79.74% <100%> (+2.62%)` | :arrow_up: |
| [requests/exceptions.py](https://codecov.io/gh/requests/requests/pull/4581/diff?src=pr&el=tree#diff-cmVxdWVzdHMvZXhjZXB0aW9ucy5weQ==) | `100% <100%> (ø)` | :arrow_up: |
| [requests/adapters.py](https://codecov.io/gh/requests/requests/pull/4581/diff?src=pr&el=tree#diff-cmVxdWVzdHMvYWRhcHRlcnMucHk=) | `93.11% <100%> (-0.16%)` | :arrow_down: |
| [requests/api.py](https://codecov.io/gh/requests/requests/pull/4581/diff?src=pr&el=tree#diff-cmVxdWVzdHMvYXBpLnB5) | `81.81% <0%> (-0.8%)` | :arrow_down: |
| ... and [6 more](https://codecov.io/gh/requests/requests/pull/4581/diff?src=pr&el=tree-more) | |

------

[Continue to review full report at Codecov](https://codecov.io/gh/requests/requests/pull/4581?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/requests/requests/pull/4581?src=pr&el=footer). Last update [9c6bd54...1b7fc1d](https://codecov.io/gh/requests/requests/pull/4581?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).


@gahancorpcfo it looks like you still have some extra code in cookies.py. Would you mind removing that? `git add -p` will allow you to easily select the chunks you need.

@nateprewitt 

The extra code has been removed from the commit now, it should be good to go.

@gahancorpcfo thanks so much for seeing this through! We have an issue right now with our Windows build environment that's failing on Python 3.4, so I'm going to wait to merge this. We'll get this in once that's resolved.

As for the issue, it looks like https://github.com/jonparrott/cmarkgfm has had issues with Windows wheel builds recently between versions 0.2.0 and 0.4.0. I'll need to reach out to Jon to see if this is supposed to be fixed. That is unless someone else would like to spend some time digging and opening a relevant issue with the repository.

Just as a follow up note, jonparrott/cmarkgfm#2 is the issue we're hitting right now. Until a wheel is built for 3.4 (hopefully in the next week), 3.4 won't be passing on Windows under appveyor.

@nateprewitt Thanks for the heads up!  I'm happy to help. 

Sorry it took so long, but 3.4 wheels should now be available for Windows. :)

No worries, thanks @jonparrott!

In an effort to clean up the issue tracker to only have issues that are still relevant to the project we've done a quick pass and decided this issue may no longer be relevant for a variety of potential reasons, including:

* Applies to a much older version, unclear whether the issue still applies.
* Change requires a backwards incompatible release and it's unclear if the benefits are worth the migration effort from the community.
* There isn't a clear demand from the community on the change landing in Requests.

If you think the issue should remain open, please comment so below or open a new issue and link back to the original issue. Again, thank you for opening the issue and for the discussion, it's much appreciated.