hal_json_style: implement default_to_now on date time fields

refs !73  72db2000

bt5/erp5_hal_json_style/SkinTemplateItem/portal_skins/erp5_hal_json_style/ERP5Document_getHateoas.xml

@@ -239,6 +239,8 @@ def renderField(traversed_document, field, form_relative_url, value=None, meta_t
       "hidden_day_is_last_day": field.get_value(\'hidden_day_is_last_day\'),\n
     }\n
     date_value = getFieldDefault(traversed_document, field, result["key"], value)\n
+    if not date_value and field.get_value(\'default_now\'):\n
+      date_value = DateTime()\n
     if same_type(date_value, DateTime()):\n
       # Serialize DateTime\n
       date_value = date_value.rfc822()\n