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, 3 deletions

diff --git a/actioncable/lib/action_cable/channel/test_case.rb b/actioncable/lib/action_cable/channel/test_case.rb
index c4cf0ac0e7..19d6f1cccf 100644
--- a/actioncable/lib/action_cable/channel/test_case.rb
+++ b/actioncable/lib/action_cable/channel/test_case.rb
@@ -300,9 +300,7 @@ module ActionCable
           def broadcasting_for(stream_or_object)
             return stream_or_object if stream_or_object.is_a?(String)
 
-            self.class.channel_class.broadcasting_for(
-              [self.class.channel_class.channel_name, stream_or_object]
-            )
+            self.class.channel_class.broadcasting_for(stream_or_object)
           end
       end