browser can't download source map after reloading a page

Steps to Reproduce

1. test this case with 8.5.0.FL.20170920-Eval
2. load a simple zul like

   <zk>
      <button label="test"/>
   </zk>
   

3. open chrome developer tool, we can see source map files
   
4. reload the page again

Current Result

• can't see any source map in developer tool
• server console shows errors:
  Sep 21, 2017 4:42:08 PM org.zkoss.zk.ui.http.WpdExtendlet mergeJavaScript
  WARNING: Failed to merge the source map: /js/zul.wpd
  Sep 21, 2017 4:42:08 PM org.zkoss.zk.ui.http.WpdExtendlet mergeJavaScript
  WARNING: Failed to merge the source map: /js/zul.wgt.wpd
  Sep 21, 2017 4:42:08 PM org.zkoss.zk.ui.http.WpdExtendlet$WpdContent toByteArray
  SEVERE: frozen.js not found
  Sep 21, 2017 4:42:08 PM org.zkoss.zk.ui.http.WpdExtendlet mergeJavaScript
  WARNING: Failed to merge the source map: /js/zkmax.wpd

Expected Result

users can see source map in developer tool

Debug Info