Front page not embeddable when using a static page_on_front

[dd32]

When a site is configured with a page_on_front, Canonical kicks in and redirects the user to the homepage.

This is caused by Canonical not being embed-aware.

As an example, the WordPress.org homepage should be embeddable:

$ curl -Is https://en-au.wordpress.org/embed/ | grep -E '^(HTTP|location)'
HTTP/2 301
location: https://en-au.wordpress.org/

The non-pretty embeds do however work, unfortunately WordPress doesn't link to these when rewrites are enabled:

$ curl -Is https://en-au.wordpress.org/?embed=1 | grep -E '^(HTTP|location)'
HTTP/2 200

After the PR attached to this ticket:

$ curl -ILs https://en-au.wordpress.org/embed/ | grep -E '^(HTTP|location)'
HTTP/2 200

The fix attached is a combination bugfix/enhancement, as I've fixed the bug by making canonical embed aware, so it redirects to the canonical embed location.

The PR isn't complete, as it appears to still fail on one of the test-cases I added, and appears to have altered (Potentially fixed?) the behaviour of another canonical test.

A more targeted fix would be to disable canonical when is_embed() is truthful.

Trac ticket: https://core.trac.wordpress.org/ticket/54758