Bug/multipart attachment

Re: #8

Since when people try to POST something via `attach`, it will add `attachment` instead of `form-data` in `Content-Disposition` that it always passed `RE_FIELD` regexp test.

So I basically added the condition of `When Content-Disposition is not form-data nor attachment, then skip`.
Test source is added accordingly.

Please check this, and let me know if I may need to change some coding convention!

*here's the dump of content-disposition when send via from `superagent`/`supertest`*

```javascript

request()
  .post('/upload')
  .attach('image', '/path/to/test.jpg') // => 'Content-Disposition: attachment; name="image"; filename="test.jpg"'
  .end();

```