Run zstd before brotli if both are enabled

[kamermans]

Since Safari doesn't support zstd yet, I have to keep brotli enabled, however, this module is run after brotli, so if both are supported, brotli is used. This is a problem since all browsers that support zstd also support brotli, so zstd is never actually used.

This PR reorders the module/filter before brotli so it takes priority (since it's faster and the compression is better).

I took this patch from #40 by @mklooss (thanks!)