by law to be attested.— If a document is required by law to be attested, it shall not be used as evidence until two attesting witnesses at least have been called for the purpose of proving its execution, if there be two attesting witnesses alive, and subject to the process of the Court and capable of given evidence:

Provided that it shall not be necessary to call an attesting witness in proof of the execution of any document, not being a will, which has been registered in accordance with the provisions of the Registration Act, 1908 (XVI of 1908), unless its execution by the person by whom it purports to have been executed is specifically denied.