Strip 'enclosed' trailing spaces in URLs

[jonoaldersonwp]

#20383 made improvements that strip trailing punctuation from URLs. E.g., ​https://ma.tt/2012/03/productivity-per-square-inch%20 redirects correctly to the canonical URL.

However, URLs like ​https://ma.tt/2012/03/productivity-per-square-inch%20/ (which 'enclose' the trailing space with a trailing slash) are not redirected. It, and others like it, typically return a 404 error.

This kind of 'broken link' pattern is extremely common on the web; particular as a trailing slash is often appended to a malformed URL before WP runs (e.g., via a server/htaccess/nginx configuration).

We should refine the canonical redirect logic (in redirect_canonical) to also consider and redirect these types of requests.

Considerations

• The "Remove trailing spaces and end punctuation from the path" section of redirect_canonical doesn't consider the presence of trailing slashes in the URL. This could/should be adapted to catch those.

• There might be cases where a user 'legitimately' has a permalink structure (or slug) that ends in %20 or %20/. That might(?) make a fix more complicated than just sniffing for whether the permalink structure ends with /.

• It looks like it's inconsistent in WP where %20 (and/or %20/ ) can be added to slugs or structures. It's stripped in some places, but not in others.

• Should a permalink or slug be 'allowed' to contain, or end in, a space character? If this is being stripped in some parts of WP, maybe that's a good argument to prevent it elsewhere/everywhere. In which case, fixing this becomes a lot simpler.