Serialize doc counts in Matrix-Stats module

This fixes a bug in the RunningStats class for the matrix stats aggregation module. doc counts were not being searlized which means they were only computed the first time the aggregation was computed. This was causing incorrect results when the aggregation was pulled from cache.

modules/aggs-matrix-stats/src/main/java/org/elasticsearch/search/aggregations/matrix/stats/RunningStats.java

@@ -78,6 +78,8 @@ public class RunningStats implements Writeable, Cloneable {
     @SuppressWarnings("unchecked")
     public RunningStats(StreamInput in) throws IOException {
         this();
+        // read doc count
+        docCount = (Long)in.readGenericValue();
         // read fieldSum
         fieldSum = (HashMap<String, Double>)in.readGenericValue();
         // counts
@@ -96,6 +98,8 @@ public class RunningStats implements Writeable, Cloneable {
 
     @Override
     public void writeTo(StreamOutput out) throws IOException {
+        // marshall doc count
+        out.writeGenericValue(docCount);
         // marshall fieldSum
         out.writeGenericValue(fieldSum);
         // counts