Action cache filter code incorrectly renders nil response body if response is non-cacheable

Best explained with an example - consider the simple controller below:

class ActionCachingTestController < ActionController::Base
  caches_action :four_oh_four

  def four_oh_four
    render :text => "404'd!", :status => 404
  end
end

The four_oh_four action returns a HTTP 404 response, which is non-cacheable. The Action cache filter, however, has faulty code that causes the response to be nil (basically an empty page is returned).

I've attached a patch that fixes this by continuing to render the response body in cases where an action is non-cacheable.

Tested against Rails master @ f3e9cbc69.