Return callback from RTMClient.run_on (#490)

* Return callback from `RTMClient.run_on`

* Tidy up test

* Change param name to snake_case
Names are important 😄

Author: clavin

slack/rtm/client.py

@@ -141,6 +141,7 @@ def run_on(*, event: str):
 
         def decorator(callback):
             RTMClient.on(event=event, callback=callback)
+            return callback
 
         return decorator
 

tests/rtm/test_rtm_client.py

@@ -17,6 +17,14 @@ def setUp(self):
     def tearDown(self):
         slack.RTMClient._callbacks = collections.defaultdict(list)
 
+    def test_run_on_returns_callback(self):
+        @slack.RTMClient.run_on(event="message")
+        def fn_used_elsewhere(**_unused_payload):
+            pass
+        
+        self.assertIsNotNone(fn_used_elsewhere)
+        self.assertEqual(fn_used_elsewhere.__name__, "fn_used_elsewhere")
+
     def test_run_on_annotation_sets_callbacks(self):
         @slack.RTMClient.run_on(event="message")
         def say_run_on(**payload):