setTimeout 0 never fires, under certain circumstances

There was a regression in 0.31.1 (commit af16109) which causes my test suite to hang at a certain point. I traced the issue to `lib/request_overrider.js` line 367: the callback passed to `setTimeout` is never run under some circumstances. This happens deep into my test suite, after many successful uses of `nock` (i.e. the `setTimeout` works as intended earlier in the suite). Indeed, if I run just the offending part of the suite, in isolation, then it passes. I am fairly confident that this isn't just the result of poorly written/fragile tests: I tried skipping the file and subsequent parts of the suite hangs in the same manner. Unfortunately, I don't have a simple test case to share, and I can't share my suite of tests. My guess is that this has something to do with accumulated state in `nock`, or a limitation on concurrent timers in node.js.

I noticed that changing `setTimeout 0` to `setImmediate` (or `process.nextTick`) resolves the issue in my case, but I don't have a deeper understanding of why that is or whether it's a general fix. @ierceg please review when you have a chance.