Move `channel_name` to Channel.broadcasting_for

That would allow us to test broadcasting made with channel, e.g.:

```ruby
class ChatRelayJob < ApplicationJob
  def perform_later(room, msg)
    ChatChannel.broadcast_to room, message: msg
  end
end
```

To test this functionality we need to know the underlying stream name
(to use `assert_broadcasts`), which relies on `channel_name`.

We had to use the following code:

```ruby
assert_broadcasts(ChatChannel.broadcasting_for([ChatChannel.channel_name, room]), 1) do
  ChatRelayJob.perform_now
end
```

The problem with this approach is that we use _internal_ API (we shouldn't care about `channel_name` prefix
in our code).

With this commit we could re-write the test as following:

```ruby
 assert_broadcasts(ChatChannel.broadcasting_for(room), 1) do
   ChatRelayJob.perform_now
 end
```

1 files changed, 12 insertions, 7 deletions

diff --git a/actioncable/lib/action_cable/channel/broadcasting.rb b/actioncable/lib/action_cable/channel/broadcasting.rb
index 9a96720f4a..bd900e1c65 100644
--- a/actioncable/lib/action_cable/channel/broadcasting.rb
+++ b/actioncable/lib/action_cable/channel/broadcasting.rb
@@ -12,17 +12,22 @@ module ActionCable
       module ClassMethods
         # Broadcast a hash to a unique broadcasting for this <tt>model</tt> in this channel.
         def broadcast_to(model, message)
-          ActionCable.server.broadcast(broadcasting_for([ channel_name, model ]), message)
+          ActionCable.server.broadcast(broadcasting_for(model), message)
         end
 
-        def broadcasting_for(model) #:nodoc:
+        # Returns a unique broadcasting identifier for this <tt>model</tt> in this channel.
+        def broadcasting_for(model)
+          serialize_broadcasting([ channel_name, model ])
+        end
+
+        def serialize_broadcasting(object) #:nodoc:
           case
-          when model.is_a?(Array)
-            model.map { |m| broadcasting_for(m) }.join(":")
-          when model.respond_to?(:to_gid_param)
-            model.to_gid_param
+          when object.is_a?(Array)
+            object.map { |m| serialize_broadcasting(m) }.join(":")
+          when object.respond_to?(:to_gid_param)
+            object.to_gid_param
           else
-            model.to_param
+            object.to_param
           end
         end
       end