A covenant by a grantor in a deed "that he will execute further assurances of the land as may be requisite" has the same effect as if the grantor had covenanted that he at any time on any reasonable request, at the expense of the grantee, will do any further act and execute any further instrument to perfect the grant and assure to the grantee the lands granted, or intended to be granted, as shall be reasonably required by the grantee or his attorney.