Fix accidental caching of proc objects instead of the content it represents

Ticket: https://rails.lighthouseapp.com/projects/8994

Hi Core,

after a production deploy, we noticed some images (delivered by send_file) stopped working, seemingly after a short period of activity.

After some investigation we discovered the cache (page_cache) for these images was being populated with the following string:

#<Proc:0x01a10f80@./test/../lib/action_controller/streaming.rb:91>

This caused many headaches, and we eventually deployed a fix that simply set :stream => false on the call.

The fact that this was surprising, motivated me to write the patch attached to the ticket.

Have a look, let me know what you think.

Thanks

- Tom