Do not overwrite `Primus#send`

If `primus-emitter` is loaded after `primus-rooms` the method `Primus#send` defined in `primus-rooms` is overwritten by the one defined in `primus-emitter`.
This breaks the ability to broadcast only to selected rooms.

With this fix the `primus-rooms` definition of `Primus#send` will always be used when `primus-rooms` and `primus-emitter` are used together.