Generator should not be foldable.

Author: rxin

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

@@ -46,6 +46,8 @@ trait Generator extends Expression { self: Product =>
   // however, we don't keep the output field names in the Generator.
   override def dataType: DataType = throw new UnsupportedOperationException
 
+  override def foldable: Boolean = false
+
   override def nullable: Boolean = false
 
   /**