Preserve order in multipart requests
Right now multipart data is read into a `Table` using `content-disposition`'s `name` field as key, which means that the order of parts is lost, and parts with overlapping names (or without `name` field in `content-disposition` header) collide. The easiest solution is to make `MultipartData` use `OrderedTable` instead - this way most existing code should work as intended, but restoring initial part order is made possible.
