错误'缺less必要的参数：grant_type'使用Swift 3

我正在Twitter的API，我想获得access_Token，但我得到这个错误：

{ "errors" : [ { "message" : "Missing required parameter: grant_type", "label" : "forbidden_missing_parameter", "code" : 170 } ] }. 

我的要求如下：

 let dict = ["grant_type" : "client_credentials"] requestPOSTURL("https://api.twitter.com/oauth2/token", params: dict as [String : AnyObject] , headers: ["Authorization" : "Basic " + strHeader, "Content-Type" : "application/x-www-form-urlencoded;charset=UTF-8"], success: { (json) in Helper.sharedInstance.Print(json as AnyObject) }, failure: {(error) in Helper.sharedInstance.Print(error as AnyObject) }). func requestPOSTURL(_ strURL : String, params : [String : AnyObject]?, headers : [String : String]?, success:@escaping (JSON) -> Void, failure:@escaping (Error) -> Void){ sessionManager.request(strURL, method: .post, parameters: params, encoding: JSONEncoding.default, headers: headers).responseJSON { (responseObject) -> Void in //print(responseObject) if responseObject.result.isSuccess { let resJson = JSON(responseObject.result.value!) success(resJson) } if responseObject.result.isFailure { let error : Error = responseObject.result.error! failure(error) } } } 

在这里，我正在尝试获取access_Token（OAuth），但无法获得，因为上面的错误即将到来。