fix: encoding panic on append with Diátaxis in the meta data #40

thread 'tokio-runtime-worker' panicked at /Users/andy/.cargo/registry/src/index.crates.io-6f17d22bba15001f/cross-stream-0.2.0/src/api.rs:230:10:
called Result::unwrap() on an Err value: ToStrError { _priv: () }
note: run with RUST_BACKTRACE=1 environment variable to display a backtrace