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.
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
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