Where the exercise or performance of an agency is by written instrument, the agency shall also be created by written instrument; provided, however, unless a contrary intent is expressly set forth therein, any written instrument creating an agency regardless of the formality of its execution shall conclusively be deemed to authorize the execution of instruments with the formalities necessary or appropriate to accomplish the purposes for which the agency was granted. A corporation may create an agent in its usual mode of transacting business and without its corporate seal. Any deed or other instrument executed under seal pursuant to an agency created by an act not under seal, if not otherwise required to be under seal for its validity, shall be binding upon the principal and valid as if an unsealed instrument.