[SPARK-6902] [SQL] [PYSPARK] Row should be read-only

Raise an read-only exception when user try to mutable a Row.

Author: Davies Liu <davies@databricks.com>

Closes #8009 from davies/readonly_row and squashes the following commits:

8722f3f [Davies Liu] add tests
05a3d36 [Davies Liu] Row should be read-only