The trailng slash on a pathless url is not meaningful, but we were
treating the url given to the discovery cache as if it were. In some
circumstances, such as an endpoint_override that didn't match the
found discovery document perfectly, a double-request could be made.
Normalize root urls in the caching code so that https://example.com and
https://example.com/ would be the same.
Change-Id: I70a5911cf0f213a7816fe8d58c6cca4702ff71bb