One who assumes to act as an agent is responsible to third persons as a principal for his acts in the course of his agency, in any of the following cases, and in no others:

1. When, with his consent, credit is given to him personally in a transaction;

2. When he enters into a written contract in the name of his principal, without believing, in good faith, that he has authority to do so; or,

3. When his acts are wrongful in their nature.