Add _create_record and _update_record methods to fake InstanceMethods

Needed to support Rails master and 4-1-stable. These methods are renamed there (see https://github.com/rails/rails/commit/df1df87e86a43d9e5631dd182e2e4a4d557d74c8#diff-dfeaefe6987aa11980cf54e4ee58c4a8)