Merge pull request #465 from libp2p/fix/mock-race-really

*really* fix the race in mock

p2p/net/mock/mock_stream.go

@@ -64,7 +64,7 @@ func (s *stream) Write(p []byte) (n int, err error) {
 	select {
 	case <-s.closed: // bail out if we're closing.
 		return 0, s.writeErr
-	case s.toDeliver <- &transportObject{msg: p, arrivalTime: t}:
+	case s.toDeliver <- &transportObject{msg: cpy, arrivalTime: t}:
 	}
 	return len(p), nil
 }