This doesn't cope well with gzipped, binary, or large responses. Ideally we would teach FinalDestination to safely retrieve and decode some of the response body. But for now, let's remove the broken implementation.
It's very easy to forget to add `require 'rails_helper'` at the top of every core/plugin spec file, and omissions can cause some very confusing/sporadic errors.
By setting this flag in `.rspec`, we can remove the need for `require 'rails_helper'` entirely.
