Fix the last bit preventing all the unit tests from passing in media types->plugins

The last commit was also small, so Jeff Moe gets... two!  Two sponsored commits!
Ah ah ah. </count_voice>

mediagoblin/db/mixin.py

@@ -205,7 +205,8 @@ class MediaEntryMixin(GenerateSlugMixin):
         """
         manager = hook_handle('get_media_manager', self.media_type)
         if manager:
-            return manager
+            return manager(self)
+
         # Not found?  Then raise an error
         raise FileTypeNotSupported(
             "MediaManager not in enabled types. Check media_type plugins are"