fixes for passenger in production mode

-  ENV is the correct place to look for headers under passenger
-  When we are proxying requests to candlepin, the request body sent
   to katello is not a StringIO anymore, it is a Passenger::Input::TeeInput.
   When RestClient tries to log the request it read the input and then calls
   seek(0) to reset the stream.  TeeInput has no seek() method, but does have a
   rewind method. This adds a monkey patch to translate the seek to a rewind
Merge pull request #3659 from jlsherrill/tee

fixes for passenger in production mode