update attribute of zip had the wrong default value (my fault).

git-svn-id: https://svn.apache.org/repos/asf/ant/core/trunk@269327 13f79535-47bb-0310-9956-ffa450edef68

src/main/org/apache/tools/ant/taskdefs/Zip.java

@@ -78,7 +78,7 @@ public class Zip extends MatchingTask {
     private File zipFile;
     private File baseDir;
     private boolean doCompress = true;
-    private boolean doUpdate = true;
+    private boolean doUpdate = false;
     private boolean doFilesonly = false;
     protected String archiveType = "zip";
     // For directories: