erp5_web_renderjs_ui: browser send GMT suffix

bt5/erp5_web_renderjs_ui/SkinTemplateItem/portal_skins/erp5_web_renderjs_ui/WebPage_viewAsWeb.py

+from DateTime import DateTime
+
 if REQUEST is None:
   REQUEST = context.REQUEST
 if response is None:
@@ -11,10 +13,12 @@ response.setHeader("Access-Control-Allow-Origin", "*")
 
 web_page = context
 web_section = context.getWebSectionValue()
+# Must-Revalidate caching policy uses Base_getWebSiteDrivenModificationDate
 modification_date_string = web_page.Base_getWebSiteDrivenModificationDate().rfc822()
 
-# Must-Revalidate caching policy uses Base_getWebSiteDrivenModificationDate
-if REQUEST.getHeader('If-Modified-Since', '') == modification_date_string:
+modified_since = REQUEST.getHeader('If-Modified-Since', '')
+if modified_since:
+  if DateTime(modified_since).rfc822() == modification_date_string:
     response.setStatus(304)
     return ""