KAFKA-1316 Follow-up patch for concurrent modification exception.

clients/src/main/java/org/apache/kafka/clients/producer/internals/Sender.java

@@ -14,6 +14,7 @@ package org.apache.kafka.clients.producer.internals;
 
 import java.util.ArrayList;
 import java.util.HashMap;
+import java.util.Iterator;
 import java.util.List;
 import java.util.Map;
 import java.util.Set;
@@ -146,9 +147,11 @@ public class Sender implements Runnable {
         Set<Node> ready = this.accumulator.ready(cluster, now);
 
         // remove any nodes we aren't ready to send to
-        for (Node node : ready) {
+        Iterator<Node> iter = ready.iterator();
+        while (iter.hasNext()) {
+            Node node = iter.next();
             if (!this.client.ready(node, now))
-                ready.remove(node);
+                iter.remove();
         }
 
         // create produce requests