Don't remove a trailing slash from the mount endpoint if the mount endpoint is a slash

We mount faye at the root, '/', and this worked fine in 0.8.8, but we're trying to upgrade and in 0.8.9, the `gsub` added here:

https://github.com/faye/faye/blob/master/lib/faye/adapters/rack_adapter.rb#L28

appears to have broken support for mounting faye at the root.

I tweaked the logic for the `endpoint_re` not to do the `gsub` if `@endpoint` is a "/" and added a spec.