Include manual indexed tags into post tags sitemap

Checks noindex and include status for each term.
If 'index' and 'always' are selected for a term, this will include it in the 'post_tag-sitemap.xml'.

The old validation double checked variable type with useless functions like 'is_string()' while '===' does the same.
Merge branch 'master' of https://github.com/ABInternet/wordpress-seo into ABInternet-master
Remove unneeded comment as @todo is already done (few lines above)
Merge branch 'ABInternet-master'