router: add support for inverse header matching (#3443)
router: add support for inverse header matching (#3443)

Signed-off-by: Matthew Gumport <me@gum.pt>

Mirrored from https://github.com/envoyproxy/envoy @ 3df5d57898117d4009c475046352cf0d078c9519

lyft/envoy-api,lyft/envoy-api,lyft/envoy-api