zope webdav use bobobase_modification_time for last modification date

The test was randomly failing since we were not getting last modification
date exactly the same way as the zope webdav support

product/ERP5/tests/testWebDavSupport.py

@@ -269,7 +269,7 @@ class TestWebDavSupport(ERP5TypeTestCase):
     self.assertEquals(DateTime(xml_metadata.find('{DAV:}response/'\
                       '{DAV:}propstat/{DAV:}prop/{DAV:}getlastmodified')\
                       .text).ISO8601(),
-                      document.getModificationDate().toZone('UTC').ISO8601())
+                      document.bobobase_modification_time().toZone('UTC').ISO8601())
 
   @expectedFailure
   def test_PROPFIND_on_document_bis(self):