Introduce a new method for conveniently returning all enqueued `RKObjectRequestOperation` instances that match a given HTTP method and path pattern. closes #1356

This change generalizes the functionality used internally by `cancelAllObjectRequestOperationsWithMethod:matchingPathPattern:` and provides a convenient means for determining if any operations are already enqueued and retrieving them for manipulation.

Signed-off-by: Blake Watters <blakewatters@gmail.com>