500 error when fetching non-Latin characters via `serverProxy`

[solavrc]

When extension receives responses containing Unicode characters (especially non-Latin characters like Japanese or Chinese) from a backend API via serverProxy, a 500 error occurs:

POST https://{YOUR_ORG_SUBDOMAIN}.cloud.looker.com/api/internal/extension/external_api_proxy/{YOUR_PROJECT_NAME}::{YOUR_EXTENSION_NAME}
> 500 Internal Server Error
> {error: "Uh oh! Something went wrong."}

I've created a minimal reproduction repository demonstrating this issue:
https://github.com/solavrc/looker-unicode-issue

and implemented a workaround in #44.