Fix dropped key clean up in DataCommunicatorConnector

Change-Id: Ia6c3307c2ae4128af622d2e7bccf1dcdda080671

client/src/main/java/com/vaadin/client/tokka/connectors/data/DataCommunicatorConnector.java

@@ -103,6 +103,7 @@ public class DataCommunicatorConnector extends AbstractExtensionConnector {
             for (String key : droppedKeys) {
                 dropped.set(i++, key);
             }
+            droppedKeys.clear();
 
             getRpcProxy(DataRequestRpc.class).dropRows(dropped);
         }