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, 1 insertions, 1 deletions

diff --git a/actioncable/lib/action_cable/channel/streams.rb b/actioncable/lib/action_cable/channel/streams.rb
index 81c2c38064..7e1ed3c850 100644
--- a/actioncable/lib/action_cable/channel/streams.rb
+++ b/actioncable/lib/action_cable/channel/streams.rb
@@ -99,7 +99,7 @@ module ActionCable
       # Pass <tt>coder: ActiveSupport::JSON</tt> to decode messages as JSON before passing to the callback.
       # Defaults to <tt>coder: nil</tt> which does no decoding, passes raw messages.
       def stream_for(model, callback = nil, coder: nil, &block)
-        stream_from(broadcasting_for([ channel_name, model ]), callback || block, coder: coder)
+        stream_from(broadcasting_for(model), callback || block, coder: coder)
       end
 
       # Unsubscribes all streams associated with this channel from the pubsub queue.