[FIX] web: /web/image: conforming HTTP status line

HTTP status lines are required to have both
a status code *and* a "reason phrase".

Some stricter HTTP clients / proxies choke
on status lines without "reason".

Technically, setting `response.status` is supposed
to set both at the same time, so we need to provide
both, or to set `status_code` only and let werkzeug
add the reason.  This patch does the latter.

Fixes odoo#8924

addons/web/controllers/main.py

@@ -1088,7 +1088,7 @@ def content_image(self, xmlid=None, model='ir.attachment', id=None, field='datas
         image_base64 = content and base64.b64decode(content) or self.placeholder()
         headers.append(('Content-Length', len(image_base64)))
         response = request.make_response(image_base64, headers)
-        response.status = str(status)
+        response.status_code = status
         return response
 
     # backward compatibility