If a will be attested by a person to whom, or to whose wife or husband, any beneficial interest in any estate is thereby devised or bequeathed, if the will may not be otherwise proved such person shall be deemed a competent witness; but such devise or bequest shall be void, except that, if such witness would be entitled to any share of the estate of the testator, in case the will is not established, so much of his share shall be saved to him as shall not exceed the value of what is so devised or bequeathed. In case the will be contested any such attesting witness may, at the instance of any contestant, be required, either in court or by deposition, to testify as upon, and with the effect of, cross-examination; and the giving of such testimony or testimony in rebuttal thereto by such attesting witness, shall not, if the will be established or admitted to probate, affect in any manner the devise or bequest to such attesting witness, or to the wife or husband of such witness.