update to use AutoCastInputTypes

Author: zhichao-li

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/math.scala

@@ -359,7 +359,7 @@ case class Pow(left: Expression, right: Expression)
  * Resulting characters are returned as a byte array.
  */
 case class UnHex(child: Expression)
-  extends UnaryExpression with ExpectsInputTypes with Serializable  {
+  extends UnaryExpression with AutoCastInputTypes with Serializable  {
 
   override def expectedChildTypes: Seq[DataType] = Seq(StringType)