[REV] datetime for source-only WebAsset

thinking about it, this is the *inline* data taken directly from the template,
probably makes more sense if it does not change the coalesced asset's
last_modified

bzr revid: xmo@openerp.com-20140417143656-upgj5b0syi10c9ve

openerp/addons/base/ir/ir_qweb.py

@@ -1066,7 +1066,7 @@ class WebAsset(object):
     def last_modified(self):
         if self.source:
             # TODO: return last_update of bundle's ir.ui.view
-            return datetime.datetime.utcnow()
+            return datetime.datetime(1970, 1, 1)
         return datetime.datetime.fromtimestamp(os.path.getmtime(self.filename))
 
 class JavascriptAsset(WebAsset):