Change -emit-defaults behaviour so that the flag isn't anymore needed to show default values (Proto3)

[behofer]

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Hi, I encountered the same issue as described in #97. After some debugging, I tried to call the ame service with BloomRPC GUI and default value was shown immediately. Some research lead me to -emit-defaults option which solves the issue for me. Although this is expected behaviour as explained in #97, it would be desirable to have -emit-defaults set by default and give the user the option to omit defaults if needed. This is obviously how BloomRPC and other clients/client libraries work.

Or is there anything against this approach?

Thanks!