multipart support doesnt seem to work

Say I have this code:

var request = unirest.post(endpoint);
request.part({
'Content-Disposition': 'file; filename="inflation.tiff"',
'content-type': 'image/tiff',
'Content-Transfer-Encoding' : 'base64',
body: base64Image
});

Couple of things:

1) index.js looks for "content-type" which is case sensitive.
2) This code errors out with:
/home/jnankin/node_modules/unirest/index.js:321
$this.options.multipart.push(options);
^
TypeError: Cannot call method 'push' of undefined

Looking at index.js, I don't see where $this.options.multipart would ever have been set. It's only set if !$this._multipart, but _multipart is definied by default.

Inspect index.js around the multipart handling and the failing line 321, starting with how _multipart and options.multipart are initialized. Verify that request.part accepts the shown header casing and no longer throws when adding the part; the issue is done when both reported failures are resolved.

Written by the indexing model from the issue text.