handleUndefinedResponsesMiddleware needs a good way to opt out

[mutedchaos]

As it is, when handleUndefinedResponsesMiddleware is defined as an inherent middleware it is difficult for individual routes to opt out of it; they might need to just wait until they've sent their own headers to do so.

There should be a documented convenient way to accomplish that; probably something like return handleUndefinedResponsesMiddleware.ignore