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.
I don't like having so many methods that take env as a parameter. If this was an object that already had those values, I would be for it. I did an object allocation to make sure this doesn't leak anything 

```ruby

require 'rack'

TEST_CNT = 10_000_000

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

MY_HASH = {big: "hash", random: "values", maybe: "gets_coppied", i: "dontknow"}

MYPROC = Proc.new do
  can_gzip?(MY_HASH )
end

GC.start
GC.disable
start = ObjectSpace.count_objects
TEST_CNT.times { MYPROC.call }
finish = ObjectSpace.count_objects
GC.enable
finish.each do |k,v|
  puts k => (v - start[k]) / TEST_CNT.to_f
end
```

It doesn't. It is still 2 more method dispatches that have to be performed, but if that turned into a performance problem Ruby is in a lot worse shape than this PR. 

The net of this is a +7 line change for what equates to only 2 lines of logic. I'm leaning towards not doing the method extraction.