If the dvcs_feature is disabled and a request is made to the review

request WebAPI resource with the only-fields/only_fields query/post

parameter set to a subset of fields that does not include the

created_with_history field, the server would raise an exception and

return a HTTP 500. This was because the field was already excluded from

the set of serialized fields.
We now are more defensive about removing this field so as not to raise a

KeyError if the field is not in the dictionary.

Ran unit tests.