It shall not be necessary, on the trial of any cause arising under habeas corpus, to make up a written issue, though it may be done by the applicant for the writ. He may except to the sufficiency of, or controvert the return or any part thereof, or allege any new matter in avoidance. If written denial on his part be not made, it shall be considered, for the purpose of investigation, that the statements of said return are contested by a denial of the same; and the proof shall be heard accordingly, both for and against the applicant for relief.Acts 1965, 59th Leg., vol. 2, p. 317, ch. 722.