set "multipart/mixed" to Content-Type for a mail with attachments
minor cosmetic correction
compare codec objects to accept charset name differences between Python 2.x and 3.x
Use non-multipart message when a message has only one part
Added support for RFC2047 style encoding

Added RFC2047 style encoding mode to allow to send mail to
services and MUAs which don't understand RFC2231 style encoding,
minor cosmetic correction
Added tests for RFC2231/RFC2047 mode
Merge branch 'multipart_mixed'