RawText body splitting incorrect

[linus-hologram]

According to RFC 5321, each line of text found inside an SMTP message body should be no longer than 1000 bytes, including the CRLF sequence.

Hence, the CRLF sequence must follow no later than 998 ASCII characters after the start of a new line. If that is not the case, the line has to be split by inserting a CRLF sequence.

Work on this is to be done inside RawTextMail's getSplitTextBody() method.