Fix regression in behavior of passing wrapper to Foo.view

As test shows, `Foo.view('test/view', include_docs=True, wrapper=custom_wrapper)`, fails on an assertion error. This regression was introduced in the same large refactor commit I referenced earlier: https://github.com/benoitc/couchdbkit/commit/b88a6c4da90667cc5f0d8c7df9bcdc826b66dabd