do not include the final line ending in part contents

When the part ends with:

  ...
  The end.
  --boundary-string

The CRLF between "end." and "--b" is only there as required by MIME.
It is not part of the body content.  It should not be considered
part of the body.

[rt.cpan.org #80538]

This commit message was rewritten by rjbs.
changelog for body-ending crlf change