fix brotli responses being double-compressed

[dwickern]

Related to fastify/fastify-static#158

When used with fastify-static's preCompressed option, this plugin will double-compress .br files.

Currently this plugin avoids double-compressing for gzip and deflate, by inspecting the magic number in few bytes of the stream. However Brotli doesn't have a magic number to look for.

fastify-compress/index.js

Lines 491 to 494 in 714e4a9

	switch (isCompressed(data)) {
	case 1: return swap(null, new Minipass())
	case 2: return swap(null, new Minipass())
	}

fastify-compress/index.js

Lines 460 to 465 in 714e4a9

	function isCompressed (data) {
	if (isGzip(data)) return 1
	if (isDeflate(data)) return 2
	if (isZip(data)) return 3
	return 0
	}

Instead we can check for an existing Content-Encoding header on the reply. This has the extra advantage that we don't need to strip off the Content-Length header.

Checklist

• run npm run test and npm run benchmark
• tests and/or benchmarks are included
• documentation is changed or added
• commit message and code follows the Developer's Certification of Origin
  and the Code of conduct

[mcollina]

Should the new block be after setting the setVaryHeader.

[dwickern]

Shouldn't fastify-static be responsible for setting the Vary header?

[mcollina]

I don't understand the answer. Anyway I'm landing - if it becomes a problem I guess we'll get another PR.

[mcollina]

Unfortunately I landed without the test passing. I'm going to rever the commit, would you mind sending a fresh PR?

Thanks!