Changes to project metadata don't update the render parameters

Steps to reproduce:

1. In Render... dialog, check "Export Metadata"
2. Close Render dialog.
3. Open Project properties and go to Metadata tab.
4. Add/edit a title for the project.
5. Go back to Render... dialog and look at export parameters

Expected:

Export parameters should now have a "meta.attr.title.markup=..." parameters with the updated title.

Actual:

The export parameters do not change until you uncheck and check "Export Metadata" again.