Should allow absent/empty filename

As far as I can tell from reading [RFC7578 Section 4.2](https://tools.ietf.org/html/rfc7578#section-4.2) the filename is **not** mandatory. Currently multer ignores files without a filename, [relevant code here](https://github.com/expressjs/multer/blob/7ef2e81215e9b773204df44fb4aee29e58340061/lib/make-middleware.js#L97-L98).

I think that this should be addressed, though I'm not sure if we should set `originalname` to an empty string or to `undefined`...

This might possibly be a breaking change as well, I'll start by fixing it in the 2.x beta...