[IMP] web: Add support of js_class attribute for qweb views

Task 2119567

addons/web/models/models.py

@@ -1,6 +1,7 @@
 # -*- coding: utf-8 -*-
 import babel.dates
 import pytz
+from lxml import etree
 
 from odoo import _, api, fields, models
 from odoo.osv.expression import AND
@@ -202,9 +203,11 @@ class Base(models.AbstractModel):
     def fields_view_get(self, view_id=None, view_type='form', toolbar=False, submenu=False):
         r = super().fields_view_get(view_id, view_type, toolbar, submenu)
         # avoid leaking the raw (un-rendered) template, also avoids bloating
-        # the response payload for no reason
+        # the response payload for no reason. Only send the root node,
+        # to send attributes such as `js_class`.
         if r['type'] == 'qweb':
-            r['arch'] = '<qweb/>'
+            root = etree.fromstring(r['arch'])
+            r['arch'] = etree.tostring(etree.Element('qweb', root.attrib))
         return r
 
     @api.model