The wfs-states test data uses x-ogc. I haven't checked what GeoServer uses now. If we go the route of creating a separate projection object for each alias we know, we need a way to determine equality of projections. For OpenLayers' own projections, this can be done with an instanceOf check, but for proj4js projections this would not work. So projections should have an equals() method like in OpenLayers 2 if we decide to take this route.