[SPARK-12109][SQL] Expressions's simpleString should delegate to its toString.

https://issues.apache.org/jira/browse/SPARK-12109

The change of https://issues.apache.org/jira/browse/SPARK-11596 exposed the problem.
In the sql plan viz, the filter shows

![image](https://cloud.githubusercontent.com/assets/2072857/11547075/1a285230-9906-11e5-8481-2bb451e35ef1.png)

After changes in this PR, the viz is back to normal.
![image](https://cloud.githubusercontent.com/assets/2072857/11547080/2bc570f4-9906-11e5-8897-3b3bff173276.png)

Author: Yin Huai <yhuai@databricks.com>

Closes #10111 from yhuai/SPARK-12109.

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

@@ -207,12 +207,13 @@ abstract class Expression extends TreeNode[Expression] {
     }.toString
   }
 
-
   private def flatArguments = productIterator.flatMap {
     case t: Traversable[_] => t
     case single => single :: Nil
   }
 
+  override def simpleString: String = toString
+
   override def toString: String = prettyName + flatArguments.mkString("(", ",", ")")
 
   /**

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

@@ -71,9 +71,6 @@ case class WindowSpecDefinition(
     childrenResolved && checkInputDataTypes().isSuccess &&
       frameSpecification.isInstanceOf[SpecifiedWindowFrame]
 
-
-  override def toString: String = simpleString
-
   override def nullable: Boolean = true
   override def foldable: Boolean = false
   override def dataType: DataType = throw new UnsupportedOperationException

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/trees/TreeNode.scala

@@ -380,7 +380,7 @@ abstract class TreeNode[BaseType <: TreeNode[BaseType]] extends Product {
   /** Returns a string representing the arguments to this node, minus any children */
   def argString: String = productIterator.flatMap {
     case tn: TreeNode[_] if containsChild(tn) => Nil
-    case tn: TreeNode[_] => s"(${tn.simpleString})" :: Nil
+    case tn: TreeNode[_] => s"${tn.simpleString}" :: Nil
     case seq: Seq[BaseType] if seq.toSet.subsetOf(children.toSet) => Nil
     case seq: Seq[_] => seq.mkString("[", ",", "]") :: Nil
     case set: Set[_] => set.mkString("{", ",", "}") :: Nil