[SPARK-9683] [SQL] copy UTF8String when convert unsafe array/map to safe

When we convert unsafe row to safe row, we will do copy if the column is struct or string type. However, the string inside unsafe array/map are not copied, which may cause problems.

Author: Wenchen Fan <cloud0fan@outlook.com>

Closes #7990 from cloud-fan/copy and squashes the following commits:

c13d1e3 [Wenchen Fan] change test name
fe36294 [Wenchen Fan] we should deep copy UTF8String when convert unsafe row to safe row