Whenever any deed of conveyance or other instrument of writing has been heretofore executed or acknowledged, or both, under any power sufficiently authorizing the same, which power has been recited in said deed or other instrument, shall have been informally executed by an attorney in his own name, reciting his authority, instead of being executed in the name of the principal or principals, such deed or other instrument shall be taken to be of the same validity and effect as if executed in the name and behalf of the principal or principals as a party or parties thereunto: Provided, That no case heretofore decided judicially shall be affected by this act.