HEAD WAI requests should match GET path entries in the API

[fiadliel]

Servant responds to HEAD requests with the result of the GET path (minus the body), since haskell-servant/servant@050aa21

If the WAI verb is HEAD, and the path verb is GET, this path should match.

e.g.

instance ReflectMethod method => HasEndpoint (Verb method status cts a) where
    getEndpoint _ req =
      if requestMethod req == method || (requestMethod req == methodHead && method == methodGet) then
        Just (APIEndpoint [] method)
      else
        Nothing

    enumerateEndpoints _ = [APIEndpoint mempty method]
      where method = reflectMethod (Proxy :: Proxy method)

Though actually, this is incomplete; a GET route should also register a HEAD route in enumerateEndpoints, and the WAI verb should be used in preference to the route verb in the result of getEndpoint.