Also added API docs regarding header encoding, incl support for RFC 5987
for filename fields in Content-Disposition headers in multipart/form-data
parts, incl. an example of an upload request with a utf-8 encoded filename.
Solves an issue with clients encoding filenames in multipart
headers in ISO-8859-1, causing an HTTP 500 response code.
This change makes ISO-8859-1 encoded headers work, sends
a 400 Bad Request instead of 500 Internal Server Error if the
request multipart headers cannot be decoded as either UTF-8
or ISO-8859-1, defines UTF-8 content type for multipart text
fields in rebuilt body and also adds support for RFC 5987 for the
multipart file upload "filename" header component.
