make Memory datastore return path where data was saved to

stix2/datastore/__init__.py

@@ -309,7 +309,7 @@ class DataSource(with_metaclass(ABCMeta)):
         """
 
     def creator_of(self, obj):
-        """Retrieve the Identity refered to by the object's `created_by_ref`.
+        """Retrieve the Identity referred to by the object's `created_by_ref`.
 
         Args:
             obj: The STIX object whose `created_by_ref` property will be looked

stix2/datastore/memory.py

@@ -221,6 +221,7 @@ class MemorySink(DataSink):
         with io.open(path, "w", encoding=encoding) as f:
             bundle = bundle.serialize(pretty=True, encoding=encoding, ensure_ascii=False)
             f.write(bundle)
+        return path
     save_to_file.__doc__ = MemoryStore.save_to_file.__doc__