Super minor: Add require for mergeCombiners in combineByKey

We changed the behavior in 0.9.0 from requiring that mergeCombiners be null when mapSideCombine was false to requiring that mergeCombiners *never* be null, for external sorting. This patch adds a require() to make this behavior change explicitly messaged rather than resulting in a NPE.

Author: Aaron Davidson <aaron@databricks.com>

Closes #623 from aarondav/master and squashes the following commits:

520b80c [Aaron Davidson] Super minor: Add require for mergeCombiners in combineByKey

core/src/main/scala/org/apache/spark/rdd/PairRDDFunctions.scala

@@ -77,6 +77,7 @@ class PairRDDFunctions[K: ClassTag, V: ClassTag](self: RDD[(K, V)])
       partitioner: Partitioner,
       mapSideCombine: Boolean = true,
       serializerClass: String = null): RDD[(K, C)] = {
+    require(mergeCombiners != null, "mergeCombiners must be defined") // required as of Spark 0.9.0
     if (getKeyClass().isArray) {
       if (mapSideCombine) {
         throw new SparkException("Cannot use map-side combining with array keys.")