Call set method in set_default

The default value format may not be the internal format for _values like a
string for Reference field instead of a list.

issue7514
review42241002

src/model.js

@@ -1407,7 +1407,7 @@
             return this.get(record);
         },
         set_default: function(record, value) {
-            record._values[this.name] = value;
+            this.set(record, value);
             record._changed[this.name] = true;
         },
         set_on_change: function(record, value) {