use rspec "subject"

also fix some whitespace and normalize/fix variable names
check for the actual value of the header
optionally copy heroku-request-id into x-request-id

* https://github.com/rails/rails/blob/master/actionpack/lib/action_dispatch/middleware/request_id.rb
* http://guides.rubyonrails.org/3_2_release_notes.html#action-dispatch
* http://blog.rubyrockers.com/2011/10/x-request-id-tracking-taggedlogging-rails/
A little bit of spec cleanup.
Update the README to include info about request ID replication.