I receive a string that contains valid JSON from another service. I would like to just forward this string with Nancy but also set the content-type to \"application/json\" which
Pretty much the way you do it. You could do
var response = (Response)myJsonString;
response.ContentType = "application/json";
You could just create an extension method on IResponseFormatter and provide your own AsXXXX helper. With the 0.8 release there will be some extensions on the response it self so you can do stuff like WithHeader(..), WithStatusCode() etc-