Allow setting the default character set in multipart/form-data provider

The current implementation of the multipart/form-data provider conforms to RFC2046, assuming US-ASCII in absence of a charset parameter within the content type header. Other character sets must be specified explicitely, e.g.

Content-type: text/plain; charset=utf-8

However, current browsers use the encoding of the page containing the form (e.g. utf-8) and do not send this header upon form submission. So any non-US-ASCII text is garbled.

Browser-based form submission is a common need in JavaScript REST clients to support file upload.

Apache FileUpload, for example, allows the setting of the default character set.