[PATCH] Fix accidental caching of proc objects instead of the content it represents

Streaming content will return a proc that rack handles just fine, but cache_page expects a string.

This exhibits itself by caching the following instead of the expected file contents:

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

This can be avoided by setting :stream => false when calling send_file. But the problem only became apparent after we got to production... which is too late IMHO.

It should be noted that this is not an issue in 3.0, due to some more major changes in the rack stack (I think). The patch when applied to 3.0 does not cause any test fails, but neither does just applying the test. Maybe a reversion should be applied to the 3-0 branch?

Patch with tests attached.