No appointment made by will, in the exercise of any power, shall be valid unless the same be so executed that it would be valid for the disposition of the property to which the power applies, if it belonged to the testator; and every will so executed shall be a valid execution of a power of appointment by will, notwithstanding the instrument creating the power expressly requires that a will made in execution of such power shall be executed with some additional or other form of execution or solemnity.