Patch for serious nil response bug when Action caching responses that are not cacheable

[Cheah_Chu_Yeow]

Hi guys,

I've fixed a pretty annoying bug in Action caching that is detailed
here:
https://rails.lighthouseapp.com/projects/8994/tickets/6480-action-cache-filter-code-incorrectly-renders-nil-response-body-if-response-is-non-cacheable

Would appreciate testers or a direct commit if it checks out

Cheers,
Chu Yeow