fix deadlock; recursive locking (ThreadError)
issue raised at https://github.com/romanbsd/heroku-deflater/issues/10
serve_zipped_assets.rb is drawn from github.com/mattolson/heroku_rails_deflate
porting over this fix:
https://github.com/mattolson/heroku_rails_deflate/commit/d17080eb2477dadbe5345cd51c0ef4063022f39a