Fix serialization of MutablePair. Also provide an interface for easy updating.

Author: Michael Armbrust <michael@databricks.com>

Closes #141 from marmbrus/mutablePair and squashes the following commits:

f5c4783 [Michael Armbrust] Change function name to update
8bfd973 [Michael Armbrust] Fix serialization of MutablePair.  Also provide an interface for easy updating.

core/src/main/scala/org/apache/spark/util/MutablePair.scala

@@ -25,10 +25,20 @@ package org.apache.spark.util
  * @param  _2   Element 2 of this MutablePair
  */
 case class MutablePair[@specialized(Int, Long, Double, Char, Boolean/*, AnyRef*/) T1,
-                      @specialized(Int, Long, Double, Char, Boolean/*, AnyRef*/) T2]
+                       @specialized(Int, Long, Double, Char, Boolean/*, AnyRef*/) T2]
   (var _1: T1, var _2: T2)
   extends Product2[T1, T2]
 {
+  /** No-arg constructor for serialization */
+  def this() = this(null.asInstanceOf[T1], null.asInstanceOf[T2])
+
+  /** Updates this pair with new values and returns itself */
+  def update(n1: T1, n2: T2): MutablePair[T1, T2] = {
+    _1 = n1
+    _2 = n2
+    this
+  }
+
   override def toString = "(" + _1 + "," + _2 + ")"
 
   override def canEqual(that: Any): Boolean = that.isInstanceOf[MutablePair[_,_]]