default content-type header trigger preflight

[qkang07]

 // Case 2: The stringified body.
    // The string data does not need to be stringified again (this behavior follows Miniprogram).
    // Send the data directly with a custom Content-Type, defaulting to ApplicationJson.
    else if (typeof data === 'string') {
      xhr.setRequestHeader(CONTENT_TYPE, contentType || APPLICATION_JSON);
      xhr.send(data);
    }

默认添加 content-type header 会触发 preflight，如果后端没处理好会跨域