Enabled sum-types tests

Contrary to appearances, auto codec derivation seems to work for sum types.

But to make the tests work, I had to move the definitions of the sum type outside the test object. This may be loosely related to [this issue](https://github.com/milessabin/shapeless/issues/212) (although the latter's fix doesn't solve the former).

Another possible workaround would be to disable auto codec derivation for sum types, by deriving shapeless' `LabelledProductTypeClass` instead of `LabelledTypeClass`. And because of [this other possible issue](https://github.com/milessabin/shapeless/issues/257), this would solve the second point of https://github.com/argonaut-io/argonaut/issues/129 at the same time.