[SPARK-4052][SQL] Use scala.collection.Map for pattern matching instead of using Predef.Map (it is scala.collection.immutable.Map)

Please check https://issues.apache.org/jira/browse/SPARK-4052 for cases triggering this bug.
I think it's better to use `scala.collection.Map` explicitly in the code below, and add comment to explain. Another reason that makes putting this line here dangerous is that imports can be easily reorganized automatically by IDEs, which are sometimes not smart enough.