Cover an edge case of attributes() ignore the class attr_accessor for Entity

If an entity class has both attributes (defined with attributes() DSL) and its own attr_accessor
the attr_accessor should be initialized upon the contruction of the object.

Besides, a change in attributes() DSL constructor to raise exceptions when an invalid attributes is assign
through constructor params
@AlfonsoUceda because the test is no longer valid. For consistency with our default constructor, the constructor of attributes() DSL should raise error if there are no accessor responding.