Add `term_taxonomy_id` to available fields in `get_term_by()`. Adds unit test.

Props jchristopher.
Fixes #21651.



git-svn-id: https://develop.svn.wordpress.org/trunk@25334 602fd350-edb4-49c9-b593-d223f7449a82
Don't call `sanitize_title_with_dashes( $taxonomy )` in `register_taxonomy()` unless `$args['rewrite']['slug']` is empty.

Props SergeyBiryukov.
Fixes #23668.



git-svn-id: https://develop.svn.wordpress.org/trunk@25335 602fd350-edb4-49c9-b593-d223f7449a82