express 4.10.2 issue with sinon fake timers

Problem

When calling Sinon.JS's `useFakeTimers` with the most recent version of Express, requests sent to the app aren't answered. This is caused by the commit titled "Correctly invoke async router callback asynchronously", which changed line 186 of `lib/router/index.js` from `return done(layerError)` to `setImmediate(done, layerError)`. When using fake timers, functions run using `setImmediate` don't execute until one explicitly ticks forward the clock. It could be argued this isn't really a problem with Express, and rather the solution is to be sure to tick whenever one issues a request to the app. However, it's not entirely clear to me how this could be done, since one would have to know when Express got to that `setImmediate` call and only then tick. Further, even if it is possible, it's doubtful many existing tests do this, meaning this breaks a lot of existing code. Presumably this is also a problem with any other framework that manipulates `setImmediate`, not just Sinon.