Fix magn5407: GroupByKey fails in 0.7.3 

Fix defect [MAGN5407: GroupByKey fails in 0.7.3 ] (http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-5407)

This is because an internal function that used in ``GroupByKey`` should be ``__Apply()`` instead of ``Apply()``. 

@sharadkjaiswal  please review the change.