Supporting method override for JSONP (like x-http-method-override) in a NancyFx APi

NancyContextExtensions.cs

public static class NancyContextExtensions
    {
        private static readonly string[] AllowedOverrides = new [] { "POST", "PUT", "DELETE", "PATCH" };
        private static readonly string[] OverrideTriggers = new [] { "_method", "callback" };
        
        /// <summary>
        /// This method checks for JSONP override
        /// in the contexts Request item. Useful for handling unsupported verbs
        /// from a javascript client. 
        /// 
        /// If it contains '_method' and a 'callback' the request
        /// is changes the request to match the method. It also transfers 
        /// query parameters to instead be form parameters. (expect the 'callback')
        /// </summary>
        /// <param name="context"></param>
        /// <returns></returns>
        public static Response OverrideJsonpMethod(this NancyContext context) 
        {
            if (context.Request.Method != "GET" || context.Request.Query._method == null || context.Request.Query.callback == null)
                return null;

            string method = ((string)context.Request.Query._method).ToUpper();
            if (!AllowedOverrides.Contains(method))
                return null;

            // grab the headers.
            var headers = context.Request.Headers.ToDictionary(x => x.Key, x => x.Value);
            headers["Content-Type"] = new [] { "application/x-www-form-urlencoded" };

            var url = context.Request.Url;

            // transfer the parameters in the query to the form.
            DynamicDictionary query = context.Request.Query;

            var data = string.Join("&", 
                query.Keys
                    .Where(key => !OverrideTriggers.Contains(key))
                    .Select(key => string.Format("{0}={1}", key, query[key]))
            );

            // Keep only the callback in the URL
            url.Query = string.Concat("callback=", query["callback"]);

            var bytes = Encoding.UTF8.GetBytes(data);
            var stream = new MemoryStream(bytes);
            stream.Position = 0;

            // Create a new body.
            var body = RequestStream.FromStream(stream);

            // create our new request altered request.
            var rewrittenRequest = new Request(
                method,
                url,
                body,
                headers,
                context.Request.UserHostAddress
            );

            // and replace the current.
            context.Request = rewrittenRequest;

            return null;
        }
    }