Comment 0 for bug 1838262

During ARC signing, on `dkim/arcsign.py` file an error is being thrown here:

`

sig = dkim.arc_sign(message, selector, domain, open(privatekeyfile, "rb").read(),
                   srv_id, cv, dkim.util.get_linesep(message))

`

where the last parameter is clearly meant as a line separator parameter, but the `arc_sign` method is expecting `include_headers` at that place. Prepending `linesep` seems to solve the issue.