v0.9.4

优化文件上传API功能，支持Axios的Automatic serialization to FormData用法，伪代码示例如下:

wxpay.v3.merchant.media.upload.post({
 meta: JSON.stringify({ filename, sha256 }),
 file: fs.createReadStream(localfilepath),
}, {
 meta: { filename, sha256 },
 headers: { 'Content-Type': 'multipart/form-data' },
});