MAPREDUCE-7162. TestEvents#testEvents fails. Contributed by Zhaohui Xin.

(cherry picked from commit 1aad99a71813660b83628cacfed393d0b3a123cc)

hadoop-mapreduce-project/hadoop-mapreduce-client/hadoop-mapreduce-client-core/src/main/java/org/apache/hadoop/mapreduce/jobhistory/EventWriter.java

@@ -94,6 +94,7 @@ public class EventWriter {
     wrapper.setEvent(event.getDatum());
     writer.write(wrapper, encoder);
     if (this.jsonOutput) {
+      encoder.flush();
       out.writeBytes("\n");
     }
   }