Asp .net Mvc 4 Proxy Server/Controller (For help with Cross Domain Request)

Global.asax

public class WebApplication : System.Web.HttpApplication
{
    protected void Application_Start()
    {
        RouteTable.Routes.MapRoute("HttpProxy", "proxy/{*path}", new { controller = "Proxy", action = "Http" })
    }
}

init.coffee

window.su = {} # declare app namespace

#We only need proxy for IE as other browsers support CORS

su.initServerUrl = (url) ->
  if $.browser.msie && parseInt($.browser.version, 10) < 10
    su.serverUrl = '/proxy'
  else
    su.serverUrl = url  # e.g. http://api.mydataserver.com

ProxyController.cs

public class ProxyController : System.Web.Mvc.Controller
{
    protected string svcUrl = WebConfigurationManager.AppSettings["SvcUrl"];

    public ActionResult Http()
    {  
        return Content(grabContent(HttpContext.Request.Url.PathAndQuery.Replace("/proxy","")));
    }

    /// <see>http://stackoverflow.com/questions/3447589/copying-http-request-inputstream</see>
    private string grabContent(string url)
    {
        string content;

        // Create a request for the URL. 		
        var req = HttpWebRequest.Create(svcUrl+url);
        req.Method = HttpContext.Request.HttpMethod;

        //-- No need to copy input stream for GET (actually it would throw an exception)
        if (req.Method != "GET")
        {
            req.ContentType = "application/json";

            Request.InputStream.Position = 0;  //***** THIS IS REALLY IMPORTANT GOTCHA
                
            var requestStream = HttpContext.Request.InputStream;
            Stream webStream = null;
            try
            {
                //copy incoming request body to outgoing request
                if (requestStream != null && requestStream.Length > 0)
                {
                    req.ContentLength = requestStream.Length;
                    webStream = req.GetRequestStream();
                    requestStream.CopyTo(webStream);
                }
            }
            finally
            {
                if (null != webStream)
                {
                    webStream.Flush();
                    webStream.Close(); 
                }
            }
        }

        // If required by the server, set the credentials.
        req.Credentials = CredentialCache.DefaultCredentials;

        // No more ProtocolViolationException!
        using (HttpWebResponse response = (HttpWebResponse)req.GetResponse())
        {
            // Display the status.
            //Console.WriteLine(response.StatusDescription);
                
            // Get the stream containing content returned by the server.
            using (Stream dataStream = response.GetResponseStream())
            {
                // Open the stream using a StreamReader for easy access.
                StreamReader reader = new StreamReader(dataStream);
                // Read the content. 
                content = reader.ReadToEnd();
            }
        }

        return content;
    }
}

Thanks a lot, this is a really useful piece of code!
If I may suggest some improvements, I think it would be better to get the content type from the request when request is not a get, like this: req.ContentType = HttpContext.Request.ContentType; instead of forcing it to application/json (this can be used to call xml web services for example, and the hard application/json may cause trouble in that case); and to set the one of the response after having got the content (and maybe to transfer some other headers as well). A simple Response.ContentType = response.ContentType; at the end of the using should do the trick.
Also, the code will fail if the url parameter is url encoded.
Starting GrabContent by something like what follows should be able to handle both cases of url-encoded and not url-encoded url parameters:
string decoded = HttpUtility.UrlDecode(url);
if (decoded != url)
{
url = decoded;
}
That's said, thanks again!

+1 to the add headers
+1 to add response content type
+1 to decoding URL
+1 to request content-type not hardcoded.

Thanks @FennNaten !!