Fix for bug in MailMessage reported on forum

Fix for bug in MailMessage reported on forum (http://pocoproject.org/forum/viewtopic.php?f=12&t=6106&sid=9283682f915ea148d86813ea140cd3c7).
MailMessage was not handling encoding when no Content-Transfer-Encodign
header was not present for MultiPart messages, resulting on an
Exception. This fix checks if the header is present before getting is
value, and just defaults to 7bit encoding as described in 
http://www.w3.org/Protocols/rfc1341/5_Content-Transfer-Encoding.html when the header is not.

I also add two new tests to cover both simple and multipart messages with no Content-Transfer-Encoding headers.