test TupleReadSupport.union with two schemas containing a map field

TupleReadSupport.union seems to produce an invalid pig schema when merging schemas with fields of type map.
The test i have added throws an exception when trying to parse the merged schema, when I believe it shouldn't.

This behaviour prevents pig from loading parquet files with different schemas but at least one common field of type map when relying on the schema-auto-merge.