[Clojure] - Add Spec Validations to the Optimizer namespace

[gigasquid]

It would be nice to add some spec validations to the optimizer namespace https://github.com/apache/incubator-mxnet/blob/master/contrib/clojure-package/src/org/apache/clojure_mxnet/optimizer.clj

So that if a user calls a function with the incorrect arguments, it will guide them to the correct form.

Using the util/validate function as in https://github.com/apache/incubator-mxnet/blob/master/contrib/clojure-package/src/org/apache/clojure_mxnet/module.clj#L186 and the correct specs would be a great addition to the project.

Addition of unit tests to test a failing case of the spec that an exception is thrown would be great too :)

[gigasquid]

Thanks for fixing this @hellonico :)