Test enum inspect display, and fix the failing test

Tests the new way that enums will show up in `#inspect` (as a result of https://github.com/seejohnrun/protobuf/commit/67524742ab0d062d9004a9ad378a19fa32d5ec44)

Also fixes the failing test, by fixing up the `Protobuf::Message` definition in the spec

@zachmargolis @tamird