fix part content-disposition

[mscdex]

For a content-type of 'multipart/form-data', the content-disposition of each child part must be 'form-data' (and not 'attachment' for files).

[jessefulton]

+1 would love to see this get into supertest. Any suggestions on a temporary workaround? (I tried forking supertest to use the latest master of superagent, but it doesn't seem to be firing the Busboy events in my API routes...)

Edit: I had another issue in my route... looks like my fork is working fine. Thanks!