解决Alamofire库在iOS7下设置Head无效的问题

代码
  private func getRequest(method: Method, _ URLString: URLStringConvertible, parameters: [String: AnyObject]? = nil) -> NSMutableURLRequest {
    let request = NSMutableURLRequest(URL: NSURL(string: URLString.URLString)!)
    request.HTTPMethod = method.rawValue
    if parameters != nil {
      request.HTTPBody = NSJSONSerialization.dataWithJSONObject(parameters!, options: nil, error: nil)
    }
    request.setValue(API_UA, forHTTPHeaderField: "User-Agent")
    request.setValue(HEADER_ACCEPT, forHTTPHeaderField: "Accept")
    request.setValue("application/json", forHTTPHeaderField: "Content-Type")
    
    return request
  }

var request: Request!
    if DeviceUtils.isIOS7() {
      request = mHttpManager.request(getRequest(method, URLString, parameters: parameters))
    } else {
      request = mHttpManager.request(method, URLString , parameters: parameters, encoding: ParameterEncoding.JSON)
    }
    //request.responseJSON....