Change header callback to bytes, but default Milter to convert

to str with surrogateescape.

miltermodule.c

@@ -674,7 +674,7 @@ milter_wrap_header(SMFICTX *ctx, char *headerf, char *headerv) {
    if (header_callback == NULL) return SMFIS_CONTINUE;
    c = _get_context(ctx);
    if (!c) return SMFIS_TEMPFAIL;
-   arglist = Py_BuildValue("(Oss)", c, headerf, headerv);
+   arglist = Py_BuildValue("(Oyy)", c, headerf, headerv);
    return _generic_wrapper(c, header_callback, arglist);
 }