Full duplex streaming doesn't seem to work

[lann]

@dicej found that full duplex streaming in a proxy-type scenario doesn't work correctly. Should be able to verify with something like the Spin "double-echo" integration test: https://github.com/spinframework/spin/blob/9bcec7ba09978b99c214c9a2e79e92de6aa1b6a9/tests/test-components/components/integration-wasi-http-streaming/src/lib.rs#L93