l s is the initial header value.  If None, the initial header
        value is not set.  You can later append to the header with .append()
        method calls.  s may be a byte string or a Unicode string, but see the
        .append() documentation for semantics.

        Optional charset serves two purposes: it has the same meaning as the
        charset argument to the .append() method.  It also sets the default
        character set for all subsequent .append() calls that omit the charset
        argument.  If charset is not provided in the constructor, the us-ascii
        charset is used both as s's initial charset and as the default for
        subsequent .append() calls.

        The maximum line length can be specified explicitly via maxlinelen. For
        splitting the first line to a shorter value (to account for the field
        header which isn't included in s, e.g. `Subject') pass in the name of
        the field in header_name.  The default maxlinelen is 78 as recommended
        by RFC 2822.

        continuation_ws must be RFC 2822 compliant folding whitespace (usually
        either a space or a hard tab) which will be prepended to continuation
        lines.

        errors is passed through to the .append() call.
        Nr