fix instantiation of embedded object in ReflectionEmbeddedProperty

Recently, ReflectionEmbeddedProperty was updated to extend ReflectionProperty.  This broke any embeddable that extends an abstract class. That's because if a property is defined in the abstract class, <code>$this->class</code> is alway the abstract class and thus cannot be instantiated. The <code>$class</code> parameter that is passed into the constructor needs to be stored, and that class should be instantiated, not the the property's declaring class.