I just add the following in App_Start / WebApiConfig.cs class in my MVC Web API project.

config.Formatters.JsonFormatter.SupportedMediaTypes
    .Add(new MediaTypeHeaderValue("text/html") );

That makes sure you get JSON on most queries, but you can get XML when you send text/xml.

If you need to have the response Content-Type as application/json please check Todd's answer below.

NameSpace is using System.Net.Http.Headers.