[io] handle indentifiers.org urls with https
Xref parsing from sbml only accept valid identifiers.org uri. Since they changed from http to https, the validity check has been modified accordingly, accepting both http and https uri. The default base uri has been switched to https in met4J too, so change has been made to the unit tests. However, please note that a sbml with http://identifiers.org uris will have them converted to https after export