The decorator for defining fields for webapi requests,

@webapi_request_fields, was written to combine properly with

@augment_method_from, allowing fields to be inherited. This can be seen in

the documentation for various resources, where the new fields are properly

combined with old ones. Unfortunately, the code that actually checks the fields

at runtime was using the wrong variable, causing it to only look at fields

defined in the most top-level method. We didn't notice this until now because

most places that use this also pass allow_unknown=True. This change fixes up

the decorator to check the combined fields lists.
There's one complication added by the fact that if this decorator is applied

multiple times, the "inner" decorator doesn't know about the outer fields. I've

solved this by adding an assumption that if a field is present in kwargs, that

we've already parsed and validated its contents.

Reproduced a regression in Review Board 2.5.10 which was caused by the

  addition of a fields decorator in a subclass that shadowed parameters from

  the superclass. After this change, I was once again able to use the

max-results and start parameters that had been hidden.
Ran unit tests.