Configuration matching is not exiting correctly on no match

[patrickvinograd]

Betamocks::Configuration#find_endpoint is trying to return silently if the request doesn't match any configured hosts/routes/etc. But it's using

return nil unless endpoints

This results in falling through to raising an ArgumentError with "Unable to uniquely identify request! with Matched endpoints: [] instead of returning silently.

But endpoints is an empty array and an empty array returns truthy in Ruby, so I don't think this is working as intended. But I'd also consider fixing this in concert with #12 so that the behavior is more clear about why a request is not being matched by betamocks.