modifed slightly the test for models::users::add, the test for 'only username already taken' and the same for 'only email already taken' were actually reusing the 'new username' / 'new email' used to test if we can add normally a user, which made that actually this new email/new user were also taken at the time of the test
added unit test to models::Users::change_password, and updated doc of this function