added exists() fmethod to MockedRedis

test/conftest.py

@@ -132,6 +132,9 @@ class MockedRedis(object):
             return None
         return value.encode('utf-8')
 
+    def exists(self, name):
+        return name in MockedRedis.db
+
     def delete(self, key):
         if isinstance(key, bytes):
             key = key.decode('utf-8')