Test for rel=canonical the same as a redirect #643
Description
Currently we test for redirects. So we can mark as an error if you are linking to a page that has a redirect. That's good.
A redirect can be implemented by using HTTP or putting it in the HTML meta.
But I also consider a rel=canonical as a redirect if the page you are accessing is not the canonical URL. This is because GitHub Pages (and maybe some other static web hosts) don't let you set redirects and your page is accessible as extensionless (preferred) as well as .html. If the canonical one is extensionless and you are linking to .html that should be detectable as an error.