[SPARK-16791][SQL] cast struct with timestamp field fails

## What changes were proposed in this pull request?
a failing test case + fix to SPARK-16791 (https://issues.apache.org/jira/browse/SPARK-16791)

## How was this patch tested?
added a failing test case to CastSuit, then fixed the Cast code and rerun the entire CastSuit

Author: eyal farago <eyal farago>
Author: Eyal Farago <eyal.farago@actimize.com>

Closes #14400 from eyalfa/SPARK-16791_cast_struct_with_timestamp_field_fails.