Multipart request (formData) can raise double callback

Hi,

I have encountered a problem with supertest, in the case we do request with attach (multipart) it will use a formData instance and this instance will be pipped to req : https://github.com/visionmedia/superagent/blob/master/lib/node/index.js#L1017.

But if for some reason the file is not completely uploaded and we send back a response (because of an error for example). This part of the code will be reached:

https://github.com/visionmedia/superagent/blob/master/lib/node/index.js#L862

And self.callback will be called . The connection will be closed, so req will raise an error event

[Error: write ECONNRESET] code: 'ECONNRESET', errno: 'ECONNRESET', syscall: 'write' (https://github.com/visionmedia/superagent/blob/master/lib/node/index.js#L731).

In the end the callback is called a second time.

Maybe I'm doing something wrong, Is this behaviour intended ?