title: Only allow Nancy to return json or Xml and 406 when accept header is html
body: <p>I'm writing a Nancy endpoint and I want to do something that I think should be really simple. I want to support returning the content in either json or xml but when html or any other type is requested to return a 406 Not supported. I can easily force either XML or JSON only, and I guess I could do and if (accept is html) return 406  but I would assume that there is some support for this in the content Negotiation support. </p>
  
  <p>Can anybody shed any light?</p>