views::get_url() to skip default arguments

I'm using a View with 2 arguments: one passed on the URL and the second is computed. When I attach a feed to this view, the feed URL contains those 2 arguments. What I really need is that the feed URL only contains the explicit argument, not the computed one.

The patch here modifies view::get_url to only include arguments that were passed on the URL. To detect them, it sets a new attribute in the argument handler called is_default during view::_build_arguments.