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 #1487 from dpursehouse/rewrite-test_mixed_case_scheme_acceptable

Rewrite test cases to remove dependency on httpbin.org and example.com