Add append option for setting response headers.

By default both express and node opt replace any header you set which makes it somewhat inconvenient when you need to append a header instead of replace one. A third parameter to `res.set` now controls whether or not the header is set or appended.

The default behavior remains the same as before (so all existing code works as usual), and the appropriate tests have been added.

One example where this behavior might be desirable is when setting cookies. You can do things like:

```javascript
res.set('Cookie', 'foo=bar', true);
```

and _NOT_ interfere with anyone else who has happened to also set a cookie for the response.