The follow redirect APIs (e.g. escalate.so) no longer works on master

Even though e.g. the escalate.so plugin sets a new Host / URL to "escalate" to upon say a 404 failure, the core does not follow the redirects, but instead just returns the 404 response.

7.1.x does not have this issue, and I'm still trying to track down what changed caused this. Assigning to me, since I have a nagging feeling this might be my fault.