-[RKPaginator cancel] invalidates objectRequestOperation.
Add test coverage for RKObjectResponseMapperOperation's HTTP method-based matching (refs #1510)

The test checks to make sure RKObjectResponseMapperOperation filters the
provided response descriptors based on the HTTP method (and not just by
URL and status code).
Merge pull request #1502 from shsteven/development

-[RKPaginator cancel] should nil out objectRequestOperation