fix: fix format exceptions in utils.logging_debug (#514)

Changes introduced in #508 caused debug logging to break. The bug is
fixed and debug logging is now turned on during testing, so we can catch
regressions.

google/cloud/ndb/utils.py

@@ -84,7 +84,10 @@ def logging_debug(log, message, *args, **kwargs):
     **kwargs)`, otherwise this is a no-op.
     """
     if DEBUG:
-        log.debug(str(message).format(*args, **kwargs))
+        message = str(message)
+        if args or kwargs:
+            message = message.format(*args, **kwargs)
+        log.debug(message)
 
 
 class keyword_only(object):

tests/conftest.py

@@ -25,6 +25,7 @@
 from google.cloud.ndb import _eventloop
 from google.cloud.ndb import global_cache as global_cache_module
 from google.cloud.ndb import model
+from google.cloud.ndb import utils
 
 import pytest
 
@@ -34,6 +35,8 @@
 except ImportError:
     import mock
 
+utils.DEBUG = True
+
 
 class TestingEventLoop(_eventloop.EventLoop):
     def call_soon(self, callback, *args, **kwargs):