make sure a cancelled RKObjectRequestOperation calls its failure block

Currently if an operation is cancelled, it is possible for neither the success or failure block to ever get called. I noticed that the other subclasses of RKObjectRequestOperation don't seem to bother checking isCancelled, and appear to always call either their success or failure blocks, so this change seemed like an appropriately simple fix.

Note that this could cause new types of failures for code that currently cancels operations but does not handle the error condition in the failure block.