for subjects, too: remove unnecessary metadata assignment to None

Using the same behavior we implemented for SubjectSet, we are removing the else clause that would set _original_metadata to None in favor of allowing init() to set it to an empty dict.

panoptes_client/subject.py

@@ -215,8 +215,6 @@ def set_raw(self, raw, etag=None, loaded=True):
         super(Subject, self).set_raw(raw, etag, loaded)
         if loaded and self.metadata:
             self._original_metadata = deepcopy(self.metadata)
-        elif loaded:
-            self._original_metadata = None
 
     def subject_workflow_status(self, workflow_id):
         """