sorting on map-value fails if map-value is not of bytearray type

When query execution plan is created by pig, it assumes the type to be bytearray because there is no schema information associated with map fields.
But at run time, the loader might return the actual type. This results in a ClassCastException.
This issue points to the larger issue of the way pig is handling types for map-value.

This issue should be fixed in the context of revisiting the frontend logic and pig-latin semantics.

This is related to PIG-880 . The patch in PIG-880 changed PigStorage to always return bytearray for map values to work around this, but other loaders like BinStorage can return the actual type causing this issue.