Add spec for Virtus::ClassMethods#attribute
Add spec for Virtus::ClassMethods#attribute

* This method does not exist within this package, but is
  inherited through Virtus::Extensions. However it relies on
  private methods in this class, so the only way to verify the
  behaviour is to have a test for this interface.

XescuGC/virtus,dslh/virtus,mallikarjunayaddala/virtus,reevoo/virtus,JustMikey/virtus,hapiben/virtus,artemeff/virtus,faucct/virtus,solnic/virtus,macressler/virtus,baojjeu/virtus