Cachify does not support Content Negotiation

[pfefferle]

Describe the bug
If a page has different views based on the Accept header, Cachify caches the output of the first request. If the request is application/json for example, Cachify always returns the JSON output on every following request.

To Reproduce
Steps to reproduce the behavior:

1. Install the ActivityPub plugin
2. Load the author page with Accept header application/activity+json
3. Load the author page again in the browser
4. See the JSON output

Expected behavior
Only cache requests with html Accept headers (https://developer.mozilla.org/en-US/docs/Web/HTTP/Content_negotiation/List_of_default_Accept_values) and return the dynamic/uncached content otherwise.

[florianbrinkmann]

Maybe we can add a check in the .htaccess like that:

<If "(%{HTTP_ACCEPT} -strmatch '*text/html*')">
    Rewrite 
</If>

Tried to test that quickly but had a setup issue with cachify :D Wanted to post that here so I do not forget that.