wrong behaviour with last boundary in TMail::Mail#read_multipart

[erroric]

Hi,
i've found issue with the multipart messages and its last boundary.
Some mail generators loose CR LF after the last boundary, and TMail does accept it as a part of content.

Problem is in a is_sep Regexp, there is an example of correct one:

/\A--#{Regexp.quote bound}(?:--[ \t](?:\n|\r\n|\r|\z)|[ \t](?:\n|\r\n|\r))/