Fix error when trying to copy a natively written Edward random variable

[dustinvtran]

An error is raised when trying to call ed.copy on a DirichletProcess, Empirical, ParamMixture, or PointMass random variable. This is noted in

• TypeError when initializing HMC for ParamMixture model #610
• https://discourse.edwardlib.org/t/cannot-copy-pointmass-random-variables/145
• https://discourse.edwardlib.org/t/confused-by-error-message-from-inference-run-for-lda-with-klqp/119

The problem is that args/kwargs are not recorded correctly during logic in the class inheritance. If they aren't, then ed.copy has trouble recreating the random variable when copying its arguments.