This reverts commit eec2d301.

This commit broke tests. You cannot have a file called "bundler" on the load path.

  * Add the Rack::Sendfile middleware
  * Make the header to use configurable via config.action_dispatch.x_sendfile_header (default to "X-Sendfile"). 
  * Add Railties tests to confirm that these work
  * Remove the :stream, :buffer_size, and :x_senfile default options to send_file
  * Change the log subscriber to always say "Sent file"
  * Add deprecation warnings for options that are now no-ops

Note that servers can configure this by setting X-Sendfile-Type. Hosting companies and those creating packages of servers specially designed for Rails applications are encouraged to specify this header so that this can work transparently.

Make filter parameters based on request, so they can be modified for anything in the middleware stack.