[SPARK-10998] [SQL] Show non-children in default Expression.toString

Its pretty hard to debug problems with expressions when you can't see all the arguments.

Before: `invoke()`
After: `invoke(inputObject#1, intField, IntegerType)`

Author: Michael Armbrust <michael@databricks.com>

Closes #9022 from marmbrus/expressionToString.

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

@@ -174,7 +174,13 @@ abstract class Expression extends TreeNode[Expression] {
     }.toString
   }
 
-  override def toString: String = prettyName + children.mkString("(", ",", ")")
+
+  private def flatArguments = productIterator.flatMap {
+    case t: Traversable[_] => t
+    case single => single :: Nil
+  }
+
+  override def toString: String = prettyName + flatArguments.mkString("(", ",", ")")
 }