Support for auto-generated Accept-Patch header

The auto-generated `OPTIONS` response works well, but it there currently isn't any option that you can set to include [Accept-Patch](http://tools.ietf.org/html/rfc5789#section-3.1) for the media type(s) that your API's `PATCH` request accepts.

Creating a middleware function like this doesn't work because the middleware executes before the auto-generated `OPTIONS` response:

``` javascript
let acceptPatch = function(req, res, next) {
if (/PATCH/.test(req.get('Allow')))
res.set('Accept-Patch', 'application/json')
next()
}
router.use(acceptPatch) // won't work because req.get('Allow') hasn't been auto-generated yet
```