[Proposal] Ability to use custom coder in channel

Hi, I’ve got from the source code that ActionCable::Server API allows to use custom coders for messages or even not use the default ActiveSupport::JSON coder at all:

# ActionCable::Server::Broadcasting

      def broadcast(broadcasting, message, coder: ActiveSupport::JSON)
        broadcaster_for(broadcasting, coder: coder).broadcast(message)
      def broadcaster_for(broadcasting, coder: ActiveSupport::JSON)
        Broadcaster.new(self, String(broadcasting), coder: coder)

But if I use broadcasting from channel I don’t have any ability to provide other coder

# ActionCable::Channel::Broadcasting

        def broadcast_to(model, message)
          ActionCable.server.broadcast(broadcasting_for(model), message)

I think it would be fine to add that ability and also ability to set custom coder for a channel and the whole app via settings.

Of course I can redefine broadcast_to method in my channel class, but in is not so clear. May be it is a lack of documentation here.