Only HTTP/2 server use lowercase HTTP headers. Use capitalised header names for HTTP/1 support
Content Types like `application/javascript; charset=utf-8` should not trigger a warning.
Matomo's HTTP::sendHttpRequest does not send the required `Accept-Encoding` header. Standards compliant HTTP servers cannot send compressed output in this case. This change does the compression check by checking for the existence of the correct `Vary` header. Standards-compliant HTTP servers should send it when compression is possible.