Content compression detection is faulty when brotli is enabled

Right now, when displaying the “gzip status” of a site, content compression is detected by making 3 HTTP requests to the site with certain headers to simulate a web request.

One issue with this is that the headers are not specific about what kind of compression is desired, and this results in a faulty status when brotli compression is enabled. brotli is considered to be superior to gzip and so whenever a webserver has it available and the client supports it, it will normally be used. The result of this is your outbound web request, which looks a bit like this when rewritten in curl:

curl --compressed -sIL -X GET "https://<home-url>/?avoid-minify=true"

Ends up returning content-encoding: br, which fails your checks and incorrectly warns the user that compression isn’t working properly.

What you should really do to fix this is to change this:

$headers   = array(
    'Content-Type' => 'text/plain',
);

To this, indicating you only accept gzip:

$headers   = array(
    'Content-Type' => 'text/plain',
    'Accept-Encoding' => 'gzip',
);

Or alternatively, modify your checks to also accept br as a valid value for the content-encoding header.