[SQL]Add base row updating methods for JoinedRow

This will be helpful in join operators.

Author: Cheng Hao <hao.cheng@intel.com>

Closes #1187 from chenghao-intel/joinedRow and squashes the following commits:

87c19e3 [Cheng Hao] Add base row set methods for JoinedRow

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

@@ -81,6 +81,18 @@ class JoinedRow extends Row {
     this
   }
 
+  /** Updates this JoinedRow by updating its left base row.  Returns itself. */
+  def withLeft(newLeft: Row): Row = {
+    row1 = newLeft
+    this
+  }
+
+  /** Updates this JoinedRow by updating its right base row.  Returns itself. */
+  def withRight(newRight: Row): Row = {
+    row2 = newRight
+    this
+  }
+
   def iterator = row1.iterator ++ row2.iterator
 
   def length = row1.length + row2.length
@@ -124,4 +136,9 @@ class JoinedRow extends Row {
     }
     new GenericRow(copiedValues)
   }
+
+  override def toString() = {
+    val row = (if (row1 != null) row1 else Seq[Any]()) ++ (if (row2 != null) row2 else Seq[Any]())
+    s"[${row.mkString(",")}]"
+  }
 }