具体原因参考于17036

四种HttpContext实例

MultipartFormDataContent : multipart/form-data

FormUrlEncodedContent : application/x-www-form-urlencoded

StringContent : application/json 等

StreamContent : binary


设置示例
 

var content = new StringContent(param, Encoding.UTF8, "application/json");
content.Headers.ContentType.CharSet = "UTF-8";