[SPARK-5606][SQL] Support plus sign in HiveContext

Now spark version is only support ```SELECT -key FROM DECIMAL_UDF;``` in HiveContext.
This patch is used to support ```SELECT +key FROM DECIMAL_UDF;``` in HiveContext.

Author: q00251598 <qiyadong@huawei.com>

Closes #4378 from watermen/SPARK-5606 and squashes the following commits:

777f132 [q00251598] sql-case22
74dd368 [q00251598] sql-case22
1a67410 [q00251598] sql-case22
c5cd5bc [q00251598] sql-case22

sql/hive/src/main/scala/org/apache/spark/sql/hive/HiveQl.scala

@@ -1103,6 +1103,7 @@ https://cwiki.apache.org/confluence/display/Hive/Enhanced+Aggregation%2C+Cube%2C
       Cast(nodeToExpr(arg), DateType)
 
     /* Arithmetic */
+    case Token("+", child :: Nil) => Add(Literal(0), nodeToExpr(child))
     case Token("-", child :: Nil) => UnaryMinus(nodeToExpr(child))
     case Token("~", child :: Nil) => BitwiseNot(nodeToExpr(child))
     case Token("+", left :: right:: Nil) => Add(nodeToExpr(left), nodeToExpr(right))