x-default hreflang tag is wrong

[joostdevalk]

Related: #990

The hreflang="x-default" line of the hreflang lines on, for instance, plugin pages, now links to the current page. So:

• if you're on https://af.wordpress.org/plugins/wordpress-seo/;
• the x-default is https://af.wordpress.org/plugins/wordpress-seo/.
• It should at least be https://wordpress.org/plugins/wordpress-seo/.

Even better would be to leave it out completely. x-default is for language redirectors, which we currently don't have.

“The x-default hreflang value signals to our algorithms that such a page doesn’t target a specific language or locale.”

​Source

[ocean90]

Fixed in [dotorg11720] (replace x-default with en) and [dotorg11721 (don't cache the en_US entry because it uses$_SERVER['HTTP_HOST']).