server: handle invalid content-type on typed routes

A request with a malformed content-type header (`content-type: boom`)
on a typed route like

    app.post({ path: '/', contentType: 'text/html' })

will cause restify to crash due to a null pointer in the negotiator
plugin. Add a failing test as a reminder to upgrade negotiatior once
it's resolved upstream [0].

[0]: https://github.com/federomero/negotiator/pull/21