Entityclass for mock

TestCase::getMockForModel always returned an object with \Cake\ORM\Entity as entityClass. 

This should be the case if the entity class file does not exists. But if it exists it should be used, unless a different entity class is defined in the TableObject. 

I have added some code that sets the right entity. This code is only executed if the default entity (\Cake\ORM\Entity) is used and no 'entityClass' is provided in the $options array.