Eliminate Rack::File headers deprecation warning

See http://intertwingly.net/projects/AWDwR4/checkdepot/section-6.1.html
rake test produces:
   "Rack::File headers parameter replaces cache_control after Rack 1.5."

Despite what the message says, it appears that the hearders parameter change
will be effective as of Rack 1.5:

https://github.com/rack/rack/blob/rack-1.4/lib/rack/file.rb#L24
https://github.com/rack/rack/blob/master/lib/rack/file.rb#L24