WP Super Cache: do not serve cached content if client doesn't accept text/html

[donnchawp]

If the page /author/username is loaded by a browser, then that page is cached. If a Fediverse client subsequently tries to access it, the cached HTML page will be served. If there is no cached page, then JSON code is served.

When serving a cached page, the plugin should make sure that the client can accept text/html.

Ref: https://wordpress.org/support/topic/wp-supercache-seems-to-ignore-non-default-accept-header/

In the future, we should cache 'application/activity+json' requests:

• Create a "sanitized content-type" for the type of request. 'text/html' vs 'application/activity+json' to start with.
• Add content-type to the cache key
• Modify the "incomplete page" check, so it only operates on HTML pages.