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.
Merge pull request #1483 from dpursehouse/httpbin-url-without-trailing-slash

Test cases fail when `HTTPBIN_URL` does not have trailing slash