Change the formdata builder in RTecaptchaValidator.cs #256

The formdata for the ReCaptCha is incorrect. Change
var formdata = String.Format(HttpUtility.UrlEncode(this.PrivateKey), HttpUtility.UrlEncode(this.Response), HttpUtility.UrlEncode(this.RemoteIP));
to
var formdata = string.Format("secret=" + this.PrivateKey + "&response=" + this.Response + "&remoteip=" + this.RemoteIP);