[SPARK-7897][SQL] Use DecimalType to represent unsigned bigint in JDBCRDD

JIRA: https://issues.apache.org/jira/browse/SPARK-7897

Author: Liang-Chi Hsieh <viirya@gmail.com>

Closes #6438 from viirya/jdbc_unsigned_bigint and squashes the following commits:

ccb3c3f [Liang-Chi Hsieh] Use DecimalType to represent unsigned bigint.

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

@@ -53,7 +53,7 @@ private[sql] object JDBCRDD extends Logging {
       signed: Boolean): DataType = {
     val answer = sqlType match {
       case java.sql.Types.ARRAY         => null
-      case java.sql.Types.BIGINT        => LongType
+      case java.sql.Types.BIGINT        => if (signed) { LongType } else { DecimalType.Unlimited }
       case java.sql.Types.BINARY        => BinaryType
       case java.sql.Types.BIT           => BooleanType // @see JdbcDialect for quirks
       case java.sql.Types.BLOB          => BinaryType