Don't bork if request attribute is not set.

djangorestframework/serializer.py

@@ -180,7 +180,7 @@ def serialize_val(self, key, obj, related_info):
             stack.append(obj)
 
         return related_serializer(depth=depth, stack=stack).serialize(
-            obj, request=self.request)
+            obj, request=getattr(self, 'request', None))
 
     def serialize_max_depth(self, obj):
         """