[SPARK-18717][SQL] Make code generation for Scala Map work with immutable.Map also

## What changes were proposed in this pull request?

Fixes compile errors in generated code when user has case class with a `scala.collections.immutable.Map` instead of a `scala.collections.Map`. Since ArrayBasedMapData.toScalaMap returns the immutable version we can make it work with both.

## How was this patch tested?

Additional unit tests.

Author: Andrew Ray <ray.andrew@gmail.com>

Closes #16161 from aray/fix-map-codegen.

(cherry picked from commit 46d30ac4)
Signed-off-by: Cheng Lian <lian@databricks.com>