Fixed a issue that if 1st character of line is '.' (dot/period), it will be deleted by SMTP server.
So, when composing email message content (plain or quoted-printable), if '.' is first character of a line, it should be duplicated.
See http://tools.ietf.org/html/rfc5321#section-4.5.2 (Transparency section of Simple Mail Transfer Protocol)

修正了当一行的首字符是小数点 '.' 时，小数点会被吃掉的问题。(192.168\n.1.2 -> 192.1681.2)