Now ActionDispatch::Static can accept HTTP headers so that developers
will have control of returning arbitrary headers like
'Access-Control-Allow-Origin' when a response is delivered. They can
be configured through `#config.public_file_server.headers`:
config.public_file_server.headers = {
"Cache-Control" => "public, max-age=60",
"Access-Control-Allow-Origin" => "http://rubyonrails.org"
}
Also deprecate `config.static_cache_control` in favor of
`config.public_file_server.headers`.
Set `config.static_index` to serve a static directory index file not
named `index`. For example, to serve `main.html` instead of `index.html`
for directory requests, set `config.static_index` to `"main"`.
