Merge pull request #9 from GregOwen/countDistinctPartial

Made SpecificRow and types serializable by Kryo

Author: marmbrus

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

@@ -57,7 +57,7 @@ s"""
 }.foreach(println)
 }}}
  */
-abstract class MutableValue {
+abstract class MutableValue extends Serializable {
   var isNull: Boolean = true
   def boxed: Any
   def update(v: Any)
@@ -197,6 +197,8 @@ class SpecificMutableRow(val values: Array[MutableValue]) extends MutableRow {
         case _ => new MutableAny
       }.toArray)
 
+  def this() = this(Seq.empty)
+
   override def length: Int = values.length
 
   override def setNullAt(i: Int): Unit = {