Allowing to post data in multipart/form-data without files?

[wesley800]

Edit: Please check #2399 for further discussion.

I've found that in encode_request, there's no seperate argument for specifying post content-type. Currently if one wants to post in multipart/form-data, one must specify at least one item in files.

def encode_request(...):
#...
    if content is not None:
        return encode_content(content)
    elif files:
        return encode_multipart_data(data or {}, files, boundary)
    elif data:
        return encode_urlencoded_data(data)
    elif json is not None:
        return encode_json(json)

I think it'd be better to allow user to select between urlencode (application/x-www-form-urlencoded) and multipart posts. How about allowing files=None, or adding a seperate option, or checking Content-Type in header?