This fixes an issue causing errors on some response objects, such as Rack::File.
The problem is that #first isn't guaranteed to be defined on the Rack response object (as is assumed in Assets.rb). You see this with Rack::File responses, for instance.
The Rack spec does guarantee an #each method though. So pull request augments the response with a #first method that uses #each.
fixes issue where causing some rackapps to fail. problem is that not …
…all responses respond to #first.