退款文档：https://pay.weixin.qq.com/wiki/doc/apiv3/apis/chapter6_1_26.shtml
httpPost.setHeader("Content-Type","application/json; charset=utf-8");
httpPost.setHeader("Accept", "application/json");
请求信息: {"reason":"测试退款","transaction_id":"xxxxxxxx","out_refund_no":"xxxxxxxxx","notify_url":"xxxxxxxxx","funds_account":"AVAILABLE","amount":{"refund":1,"total":1,"currency":"CNY"}}
问题：
编码不对导致，需要utf8
httpPost.setHeader("Content-Type","application/json; charset=utf-8");或者httpPost.setHeader("Content-Type","application/json; charset=utf8");httpPost.setHeader("Accept", "application/json");