A witness must answer questions legal and pertinent to the matter in issue, though his answer may establish a claim against himself; but he need not give an answer which will have a tendency to subject him to punishment for a felony; nor need he give an answer which will have a direct tendency to degrade his character, unless it be the very fact in issue, or to a fact from which the fact in issue would be presumed. But a witness must answer as to the fact of his previous conviction for felony.