Fixed error when class name was specified

[slywalker]

I corrected the error in this case.

$this->addBehavior('Upload', [
    'className' => 'Josegonzalez\Upload\Model\Behavior\UploadBehavior',
    'field' => []
]);

[coveralls]

Coverage remained the same at 100.0% when pulling 73d9cc9 on slywalker:fix-error-when-class-name into 90f3f8a on josegonzalez:master.