Don't start redirect loop on docs url requests

A redirect loop was occurring for some urls, including, for example, "docs.rubygems.org/read".  This was because urls starting with docs.rubygems.org were always redirected to rubygems.org, and the url "rubygems.org/read" was redirected then redirected back to "docs.rubygems.org/read".  See https://github.com/rubygems/rubygems.org/issues/716