"Show hierarchy in dropdown" doesn't work for taxonomy field views filter.

This is a bug with taxonomy's views integration.

Steps to reproduce

1. Create a vocabulary and add several terms in high depth.
2. Add this vocabulary as a term reference field at node -- article.
3. Create a view based on article. Add this term field as filter.
4. "Configure extra settings for filter criterion", Choose Dropdown and check "Show hierarchy in dropdown". Then this filter is broken. It can't be edited unless uncheck "Show hierarchy in dropdown" again.

There's another bug with this kind of select field filter. See #2012130: Regression: Views integration for "list" field types is broken.