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.