Unexposed taxo filter overlooked

This patch fixes a strange bug I ran into where a taxonomy filter was *not* exposed but had its $this->options['expose']['optional'] property come out as TRUE (possibly from a previous state of the view where the filter was indeed exposed).
In this case, the filter took no part in the final query (= no filter).

Patch modifies views_handler_filter_term_node_tid::accept_exposed_input() so that it starts with the same check that its fellow accept_exposed_input() methods on other classes.

Patch coauthored with deadmonk.