Fix meta serialization

[sagikazarmark]

Overview

Fix spec violation reported in #158

What this PR does / why we need it

Fixes #158

[sagikazarmark]

@ManInTheBox could you maybe try this fix in your environment?

[ManInTheBox]

Sure, will give give it a try and will let you know.

[ManInTheBox]

Thanks Márk, I just tested this fix and it worked fine for me! There's only one potential caveat as you've said

The important thing is that clients can interpret what's traveling over the wire.

which could be a problem with regular proto messages, but in the case of an error, the clients usually receive and interpret the errors right away from the server (request/response) so there's no way this removal of meta key could break the message contract (like it would be the case with regular proto messages). After all TwirpError is not a message 🤷

Overall, I'd say let's go merging this :)
Thanks a bunch!

[sagikazarmark]

The spec says:

On the network, an error is represented as a JSON response with those properties.

So I guess that's not really an issue in this case.

Thanks for the review and the test!