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