When the return is evidence, and when only a plea.
Section 544
SEC. 544. When the return is evidence, and when only a plea. — If it appears that the prisoner is in custody under a warrant of commitment in pursuance of law, the return shall be considered prima facie evidence of the cause of detention; but if he is restrained of his liberty by any alleged private authority, the return of the writ shall be considered only as a plea of the facts therein set forth, and the party claiming the custody shall be held to make proof of such facts; and upon the final disposition of any case, the court or judge shall make such order as to costs as the case requires.