http: testing 304-with-body behavior (#11261)
http: testing 304-with-body behavior (#11261)

Risk Level: n/a (test only)
Testing: new integration test
Docs Changes: n/a
Release Notes: n/a
Fixes #9274 as much as we can

Signed-off-by: Alyssa Wilk <b9253d1ec9cc3e9d2aab68d288ba3dab3452adf6@chromium.org>
envoyproxy/envoy,envoyproxy/envoy,lizan/envoy,lizan/envoy,envoyproxy/envoy-wasm,lyft/envoy,envoyproxy/envoy-wasm,envoyproxy/envoy,envoyproxy/envoy,lyft/envoy,envoyproxy/envoy-wasm,lyft/envoy,lizan/envoy,envoyproxy/envoy,envoyproxy/envoy,envoyproxy/envoy-wasm,envoyproxy/envoy,envoyproxy/envoy,envoyproxy/envoy,lyft/envoy,envoyproxy/envoy,envoyproxy/envoy-wasm,lizan/envoy,lizan/envoy,lyft/envoy,envoyproxy/envoy-wasm,lyft/envoy,envoyproxy/envoy,envoyproxy/envoy,envoyproxy/envoy-wasm,lizan/envoy