Enable gzip compression by default

If someone is using ActionDispatch::Static to serve assets and makes it past the `match?` then the file exists on disk and it will be served. This PR adds in logic that checks to see if the file being served is already compressed (via gzip) and on disk, if it is it will be served as long as the client can handle gzip encoding. If not, then a non gzip file will be served.

This additional logic slows down an individual asset request but should speed up the consumer experience as compressed files are served and production applications should be delivered with a CDN. This PR allows a CDN to cache a gzip file by setting the `Vary` header appropriately. In net this should speed up a production application that are using Rails as an origin for a CDN. Non-asset request speed is not affected in this PR.
Thoughts on turning this into a more expression focused method?

```ruby
def can_gzip?(env)
  gzip_encoding_accepted?(env) && gzipped_file_exists?(env)
end

def gzip_encoding_accepted?(env)
  env['HTTP_ACCEPT_ENCODING'] =~ /\bgzip\b/
end

def gzipped_file_exists?(env)
  File.exist?(File.join(@root, "#{::Rack::Utils.unescape(env["PATH_INFO"])}.gz"))
end
```