A party may not impeach a witness voluntarily called by him, except where he can show to the court that he has been entrapped by said witness by a previous contradictory statement. However, in the trial of all civil cases, either plaintiff or defendant shall be permitted to make the opposite party, or anyone for whose immediate benefit the action is prosecuted or defended, or any agent of said party, or agent of any person for whose immediate benefit such action is prosecuted or defended, or officer or agent of a corporation when a corporation is such party or for whose benefit such action is prosecuted or defended a witness, with the privilege of subjecting such witness to a thorough and sifting examination and with the further privilege of impeachment, as if the witness had testified in his own behalf and were being cross-examined.