add UndefinedDefaultValue type

Author: jaemk

graphql/utils/undefined.py

@@ -9,3 +9,19 @@ def __repr__(self):
 
 
 Undefined = _Undefined()
+
+
+class _UndefinedDefaultValue(object):
+    def __bool__(self):
+        return False
+
+    __nonzero__ = __bool__
+
+    def __eq__(self, other):
+        return isinstance(other, _UndefinedDefaultValue)
+
+    def __repr__(self):
+        return 'UndefinedDefaultValue'
+
+
+UndefinedDefaultValue = _UndefinedDefaultValue()