[SPARK-7039][SQL]JDBCRDD: Add support on type NVARCHAR

Issue:

https://issues.apache.org/jira/browse/SPARK-7039

Add support to column type NVARCHAR in Sql Server

java.sql.Types:
http://docs.oracle.com/javase/7/docs/api/java/sql/Types.html

Author: szheng79 <[email protected]>

Closes apache#5618 from szheng79/patch-1 and squashes the following commits:

10da99c [szheng79] Update JDBCRDD.scala
eab0bd8 [szheng79] Add support on type NVARCHAR

Author: szheng79

sql/core/src/main/scala/org/apache/spark/sql/jdbc/JDBCRDD.scala

@@ -60,6 +60,7 @@ private[sql] object JDBCRDD extends Logging {
       case java.sql.Types.NCLOB         => StringType
       case java.sql.Types.NULL          => null
       case java.sql.Types.NUMERIC       => DecimalType.Unlimited
+      case java.sql.Types.NVARCHAR      => StringType
       case java.sql.Types.OTHER         => null
       case java.sql.Types.REAL          => DoubleType
       case java.sql.Types.REF           => StringType