[FIX] ir_http: use meaningful ETag for images (#20283)

After setting the image on a product variant, the controller for `/web/image`
incorrectly returns the code 304 - Not Modified.  This bug occurs because
 1. the response ETag is a hash of the **last modified date** of the record;
 2. the record is actually **not** updated in that case, because
     - the image field is a non-stored computed field, and
     - its inverse method only modifies the variant's template.

Fix the issue by making the response ETag a hash of the **content**, which is
read by the controller anyway.

OPW 769302

odoo/addons/base/ir/ir_http.py

@@ -324,7 +324,7 @@ class IrHttp(models.AbstractModel):
 
         # cache
         etag = hasattr(request, 'httprequest') and request.httprequest.headers.get('If-None-Match')
-        retag = '"%s"' % hashlib.md5(last_update).hexdigest()
+        retag = '"%s"' % hashlib.md5(content).hexdigest()
         status = status or (304 if etag == retag else 200)
         headers.append(('ETag', retag))
         headers.append(('Cache-Control', 'max-age=%s' % (STATIC_CACHE if unique else 0)))