react-native – 响应本机提取而不发送正文内容

这是反应原生的代码

authenticateLogIn(){
    fetch('<URL>',{
          method: 'POST',header: {'Content-Type': 'application/json','Accept': 'application/json'},body: JSON.stringify({'username': '<username>','password':'<password>'})
        })
        .then((incoming) => incoming.json())
        .then((response) => {
          console.log(response.header);
          Alert.alert(JSON.stringify(response.body));
        })
        .done();