ActionCable: use find method when unsubscribing

If a frontend for some reason tries to unsubscribe from a non existing subscription, the following error is logged:

Could not execute command from ({"command"=>"unsubscribe", "identifier"=>"{\"channel\":\"SomeChannel\"}"}) [NoMethodError - undefined method `unsubscribe_from_channel' for nil:NilClass]

Instead, it will now properly log:

Could not execute command from ({"command"=>"unsubscribe", "identifier"=>"{\"channel\":\"SomeChannel\"}"}) [RuntimeError - Unable to find subscription with identifier: {"channel":"SomeChannel"}]

actioncable/lib/action_cable/connection/subscriptions.rb

@@ -43,7 +43,7 @@ def add(data)
 
       def remove(data)
         logger.info "Unsubscribing from channel: #{data['identifier']}"
-        remove_subscription subscriptions[data["identifier"]]
+        remove_subscription find(data)
       end
 
       def remove_subscription(subscription)