xdg-settings set default-url-scheme-handler additionally sets text/html mime type handler on Gnome 3

In addition to associating a scheme handler, xdg-settings set default-url-scheme-handler scheme app.desktop will also associate app.desktop as the default handler for text/html; this only appears to affect Gnome 3.

This appears to be a bug in set_url_scheme_handler_gnome3. It includes the line set_browser_mime "$2" || return which in the single-argument form defaults to associating app.desktop ($2) with text/html; this appears to be an accidental copy-and-paste from set_browser_gnome3.