Add copy method to PreparedRequest objects
Remove redundant session in `test_mixed_case_scheme_acceptable`
Rewrite `test_mixed_case_scheme_acceptable` to work with local httpbin

Instead of using hard-coded urls to httpbin.org, use the url defined
in `HTTPBIN_URL` replacing the scheme as necessary to test the mixed
cases.

Refs #1485
Remove redundant test case `test_uppercase_scheme`

This test verifies that an upper case scheme ('HTTP') works correctly,
but this is already tested in `test_mixed_case_scheme_acceptable`.
Rewrite `test_uppercase_scheme_redirect` to use local httpbin

Instead of redirecting to hard-coded 'example.com', use the URL
defined in `HTTPBIN_URL` with the path set to the 'html' endpoint.
Check the response URL in `test_uppercase_scheme_redirect`

Update the test to check that the URL in the response is the
one that we expect, i.e. the one it was supposed to redirect to.
Merge pull request #1490 from Lukasa/cookiedocs

Better cookie docs on sessions.
Comment markups, courtesy of @sigmavirus24
Document the Response.close() method.
Merge pull request #1476 from sigmavirus24/add_copy_to_prepared_requests

[2.0] Add copy method to PreparedRequest objects
Start work on sending cookies back

On 401's the cookies received aren't sent back to the server. See: #1336
Make the regular tests pass

I broke Digest Auth completely
Fixed finally. Also requires updated httpbin.org
Take advantage of the new copy method
Merge pull request #1494 from Lukasa/close

Document the Response.close() method.
Proxy urls should have explicit schemes.
Wrap IncompleteRead in ChunkedEncodingError.
Update urllib3 to d89d508
Fix duplication of efforts caught by @dofelw
Header keys should be native strings.

This commit follows a discussion on IRC. For more information, see the
Pull Request associated with it.