Fix flaky ndb test.

Change-Id: I2f03873cf7df4beb45c34dc84f0cce5fcbfdf57a

Author: Jon Wayne Parrott

appengine/standard/ndb/async/shopping_cart_test.py

@@ -140,4 +140,4 @@ def test_get_first_ready(testbed):
     testbed.init_urlfetch_stub()
 
     content = shopping_cart.get_first_ready()
-    assert 'html' in content
+    assert 'html' in content.lower()