RKManagedObjectRequestOperation: revert to main MOC (address #2020)

This change reverts changes introduced by [this commit](https://github.com/RestKit/RestKit/commit/129cc6f052e31116ce12502d1246f04ddd5ad85b). Currently creation of managed objects from inside private context leads to these objects are lost when used from main Managed Object Context (demonstration is in #2020).