[py3] Workarounded a Python bug in mail header encoding

    if isinstance(addr, six.string_types):

        addr = parseaddr(force_text(addr))

    nm, addr = addr

    # This try-except clause is needed on Python 3 < 3.2.4

    # http://bugs.python.org/issue14291

    try:

        nm = Header(nm, encoding).encode()

    except UnicodeEncodeError:

        nm = Header(nm, 'utf-8').encode()

    try:

        addr.encode('ascii')

    except UnicodeEncodeError:  # IDN