rfc2047: treat raw data as data in UTF-8

This is in a blind gamble that utf-8 is going to be more popular than latin1.
There's no right thing to do.
rfc2047: treat unrecognized encoding as utf-8 and don't enumerate the list of known codecs
Add support for decoding of RFC 2231 parameter names

refs #639
refactoring
Add support for encoding of RFC2231-styled parameters

refs #639
Composer: encode the file names using RFC 2231

fixes #639