Set the context's user when fetching user preferences

The call to _get_preferences makes some processing based on the user of the
context.

In order for those computations to return the right value the context must be
set when instanciating the user.

issue3068
review1711002

trytond/res/user.py

@@ -337,7 +337,7 @@ class User(ModelSQL, ModelView):
         if preferences is not None:
             return preferences.copy()
         user = Transaction().user
-        with Transaction().set_user(0):
+        with Transaction().set_user(0, set_context=True):
             user = cls(user)
         preferences = cls._get_preferences(user, context_only=context_only)
         cls._get_preferences_cache.set(key, preferences)