[FIX] models: avoid wild prefetching when serializing onchange results

We fell on a strange situation in method `onchange`, where some data was
missing from cache when serializing the result. In that case, the
prefetching
overwrites so many stuff in cache that the results of the onchanges are
lost.
Don't let that happen.

odoo/models.py

@@ -5031,10 +5031,11 @@ class BaseModel(MetaModel('DummyModel', (object,), {'_register': False})):
                 subtree = subtree[name]
 
         # collect values from dirty fields
-        result['value'] = {
-            name: self._fields[name].convert_to_onchange(record[name], record, subnames[name])
-            for name in dirty
-        }
+        with env.do_in_onchange():
+            result['value'] = {
+                name: self._fields[name].convert_to_onchange(record[name], record, subnames[name])
+                for name in dirty
+            }
 
         return result