test: increase test coverage in Router::HeaderParser (#8721)
test: increase test coverage in Router::HeaderParser (#8721)

Risk Level: low
Testing: adds test coverage
Docs Changes: n/a
Release Notes: n/a

Signed-off-by: Stephan Zuercher <12de3568c51b3e8be16eecacef359e62ce51b8cc@gmail.com>
lyft/envoy,envoyproxy/envoy-wasm,envoyproxy/envoy,lizan/envoy,eklitzke/envoy,envoyproxy/envoy-wasm,istio/envoy,envoyproxy/envoy,eklitzke/envoy,envoyproxy/envoy,envoyproxy/envoy,envoyproxy/envoy,envoyproxy/envoy,jrajahalme/envoy,lizan/envoy,envoyproxy/envoy,istio/envoy,envoyproxy/envoy-wasm,jrajahalme/envoy,lyft/envoy,lizan/envoy,envoyproxy/envoy-wasm,lyft/envoy,lyft/envoy,envoyproxy/envoy-wasm,envoyproxy/envoy,jrajahalme/envoy,istio/envoy,envoyproxy/envoy,lizan/envoy,jrajahalme/envoy,lizan/envoy,istio/envoy,envoyproxy/envoy,istio/envoy,eklitzke/envoy,istio/envoy,eklitzke/envoy,jrajahalme/envoy,eklitzke/envoy,jrajahalme/envoy,lyft/envoy,envoyproxy/envoy-wasm,envoyproxy/envoy,istio/envoy,eklitzke/envoy,envoyproxy/envoy,envoyproxy/envoy-wasm,lizan/envoy,lyft/envoy