Add copy method to PreparedRequest objects
Test cases fail when `HTTPBIN_URL` does not have trailing slash

Test cases can be run against a local httpbin server defined by
the `HTTPBIN_URL` environment variable, but it causes tests to
fail if the given URL does not end with a slash.

Ensure that the URL always ends with a slash.
Fix test cases that fail when running behind a proxy

When sending a request via `Session.send()` the proxies must be
explicitly given with the `proxies` argument.  This is not done
in the test cases, which means that they fail when run on a system
that is behind a proxy.

Update test cases to make sure the proxies are set in the sessions.
Merge pull request #1483 from dpursehouse/httpbin-url-without-trailing-slash

Test cases fail when `HTTPBIN_URL` does not have trailing slash
Merge pull request #1482 from dpursehouse/fix-testcases-with-proxy

Fix test cases that fail when running behind a proxy
Use urlparse.urljointo construct httpbin url
Derp. Use compat
Merge pull request #1486 from sigmavirus24/use_urljoin

Use urlparse.urljointo construct httpbin url
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.
Better cookie docs on sessions.
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