[SPARK-5141][SQL]CaseInsensitiveMap throws java.io.NotSerializableException

CaseInsensitiveMap throws java.io.NotSerializableException.

Author: luogankun <luogankun@gmail.com>

Closes #3944 from luogankun/SPARK-5141 and squashes the following commits:

b6d63d5 [luogankun] [SPARK-5141]CaseInsensitiveMap throws java.io.NotSerializableException

sql/core/src/main/scala/org/apache/spark/sql/sources/ddl.scala

@@ -110,7 +110,8 @@ private[sql] case class CreateTableUsing(
 /**
  * Builds a map in which keys are case insensitive
  */
-protected class CaseInsensitiveMap(map: Map[String, String]) extends Map[String, String] {
+protected class CaseInsensitiveMap(map: Map[String, String]) extends Map[String, String] 
+  with Serializable {
 
   val baseMap = map.map(kv => kv.copy(_1 = kv._1.toLowerCase))