Return target count from len(Service) calls (#874)

test/test_plugin_smtp2go.py

@@ -225,9 +225,15 @@ def test_plugin_smtp2go_attachments(mock_post):
         'user1@example.com/user2@example.com?batch=yes'.format(apikey))
     assert isinstance(obj, NotifySMTP2Go)
 
+    # objects will be combined into a single post in batch mode
+    assert len(obj) == 1
+
     # Force our batch to break into separate messages
     obj.default_batch_size = 1
-    # We'll send 2 messages
+
+    # We'll send 2 messages now
+    assert len(obj) == 2
+
     mock_post.reset_mock()
 
     assert obj.notify(