fix behavior when multiple tags are selected

[shunlog]

when multiple tags are selected, toggling the scratch off removes only the first one from its tags, so it still remains on the screen if it happened to be present in the other tags as well.

This patch makes it work as expected - all the selected tags are removed from the scratchpad's tags. To make it more consistent, maybe we should make the turn_on method to handle multiple tags as well?

BTW, I'm new to lua. How the hell is there no table.find_by_value or something? This loop stinks, but I copied the code from AwesomeWM's client:toggle_tag(t) so I assume this is the standard approach.