api: vim_set_var() should return the old value #2899

Closes #2816

src/nvim/api/private/helpers.c

@@ -146,6 +146,7 @@ Object dict_set_value(dict_T *dict, String key, Object value, Error *err)
       dict_add(dict, di);
     } else {
       // Return the old value
+      rv = vim_to_object(&di->di_tv);
       clear_tv(&di->di_tv);
     }