BUG while pass FormData `body` it will always into

[tianyingchun]

while we passed FormData in server side , it will run into these code,

opts.body = JSON.stringify(opts.body);
opts.headers.set('Content-Type', 'application/json');
opts.headers.set('Content-Length', Buffer.byteLength(opts.body));

But for form-data in server side, maybe sometimes we need to passfs.createReadStream as form-data

it should not set header 'Content-Type', 'application/json'