Fix http basic auth if protocol is missing (#22)

Basic auth only worked if a full url was supplied.  The reason is that
BasicTransport.RoundTrip checked if the resulting request URL has a the
transport's URL as a prefix.

This is fixed by setting the transport's URL to the canonical URL that is
computed a few lines earlier.

registry/registry.go

@@ -68,7 +68,7 @@ func newFromTransport(auth types.AuthConfig, transport http.RoundTripper, debug
 	}
 	basicAuthTransport := &BasicTransport{
 		Transport: tokenTransport,
-		URL:       auth.ServerAddress,
+		URL:       url,
 		Username:  auth.Username,
 		Password:  auth.Password,
 	}