Using RequestHeaderMapping works even better, because it also sets the Content-Type = application/json
in the response header, which allows Firefox (with JSONView add-on) to format the response as JSON.
GlobalConfiguration.Configuration.Formatters.JsonFormatter.MediaTypeMappings
.Add(new System.Net.Http.Formatting.RequestHeaderMapping("Accept",
"text/html",
StringComparison.InvariantCultureIgnoreCase,
true,
"application/json"));