fix delete from relation

the if-block for deleting an object from a relation was not committing; the tests were still passing because the session was still open and the next API call would pull the object out of the session, not the DB. Put a `session.remove()` before the API call to check that the removed object is actually gone from the relation, and you'll see that it's still there in the DB.

flask_restless/views.py

@@ -1124,8 +1124,8 @@ def delete(self, instid, relationname, relationinstid):
             relation.remove(relation_instance)
         elif inst is not None:
             self.session.delete(inst)
-            self.session.commit()
             is_deleted = True
+        self.session.commit()
         for postprocessor in self.postprocessors['DELETE']:
             postprocessor(is_deleted=is_deleted)
         return {}, 204