Make sure all enum values have names (including defaults)
Per protobuf spec, unset enums default to first enum value

> For enums, the default value is the first value listed in the
> enum's type definition.

Source: https://developers.google.com/protocol-buffers/docs/proto#optional
Use type_class.fetch to get a default value if it exists
Prevent using Protobuf::Enum.default, it must be on an instance
Merge pull request #203 from zachmargolis/all-enums-have-names

Make sure all enum values have names (including defaults)