purge fails for RsyncBackend if used in a multi-backend configuration

The RsyncBackend class doesn't implement _delete() to support remote deletion, however it appears that when I use an Rsync backend and a B2 (backblaze) backend, the multibackend class tries to use _delete() from Rsync. I seemed to fix this by adding a delete method to the rsync backend like so:

def _delete(self, filename):
   self._delete_list([filename, ])

but that seems rather inefficient. Perhaps the multi-backend logic needs to change?

Does the following patch seem like a good solution?

diff --git a/duplicity/backends/multibackend.py b/duplicity/backends/multibackend.py
index 7c805ef..aaffd06 100644
--- a/duplicity/backends/multibackend.py
+++ b/duplicity/backends/multibackend.py
@@ -282,7 +282,10 @@ class MultiBackend(duplicity.backend.Backend):
         for s in self.__stores:
             list = s.list()
             if filename in list:
- s._do_delete(filename)
+ if hasattr(s, '_delete_list'):
+ s._do_delete_list([filename, ])
+ elif hasattr(s, '_delete'):
+ s._do_delete(filename)
                 passed = True
                 # In stripe mode, only one item will have the file
                 if self.__mode == 'stripe':