BMailMessage

• New version of AddHeaderField() takes an extra (initial) encoding argument.  The encoding constants are defined in support/UTF8.h.

  Note that the encoding doesn't affect the header value as it's stored as in the message file's set of attributes.  For example, if you encode the "name" field as B_JIS_ENCODING, the name will be properly encoded in the message that's sent across the wires, but the "MAIL:name" attribute of the local file that holds the message will not be encoded.