The adducing of any party as a witness in any cause or proceeding by the adverse party shall not have the effect of a reference to the oath of the party so adduced: Provided always, that it shall not be competent to any party who has called and examined the opposite party as a witness thereafter to refer the cause or any part of it to his oath; and that in all other respects the right of reference to oath shall remain as at present established by the law and practice of Scotland.