[SPARK-2779] [SQL] asInstanceOf[Map[...]] should use scala.collection.Map...

[SPARK-2779] [SQL] asInstanceOf[Map[...]] should use scala.collection.Map instead of scala.collection.immutable.Map

Since we let users create Rows. It makes sense to accept mutable Maps as values of MapType columns.

JIRA: https://issues.apache.org/jira/browse/SPARK-2779

Author: Yin Huai <huai@cse.ohio-state.edu>

Closes #1705 from yhuai/SPARK-2779 and squashes the following commits:

00d72fd [Yin Huai] Use scala.collection.Map.