Allow `is_tag()` to accept `term_id`, `slug`, 'term_name` or array of any. Many other `is_*()` funcs already do this. Adds unit tests.

Props ramiy.
Fixes #18746.



git-svn-id: https://develop.svn.wordpress.org/trunk@25287 602fd350-edb4-49c9-b593-d223f7449a82