Add in GZIP compression to file serving

[vincentchu]

Compressing static assets like CSS/JS files can lead to big performance improvements. While this is typically done by a frontend proxy like Apache or Nginx, some providers like Heroku do not offer this and instead rely on the server to implement content compression.

The existing middleware API allows a developer to add compression except for files served via ServeFiles (static assets) where compression would be most useful. In this case, ServeFiles creates a separate http.Handler that is outside of the middleware stack.

To solve this problem for statically-served sites, wrap the file server's http.Handler with Gorilla's CompressionHandler. This handler just checks the request's Accept-Encoding header to enable or disable compression. See: https://godoc.org/github.com/gorilla/handlers#CompressHandler

This change should be invisible to users with the only difference being the return of gzipped responses if the appropriate Accept-Header is sent. In my production environment this led to ~2/3rds reduction in bundle sizes.

/cc @paganotoni

Fixes #1968

[paganotoni]

Tested it locally and works very nice.

[stanislas-m]

While this is typically done by a frontend proxy like Apache or Nginx, some providers like Heroku do not offer this and instead rely on the server to implement content compression.

Totally agree, but like you said, some people still want to handle that using their frontend proxy. :) Can you add an option to disable it?

[vincentchu]

Totally agree, but like you said, some people still want to handle that using their frontend proxy. :) Can you add an option to disable it?

Done!

[paganotoni]

Thanks @vincentchu!