examples: fix AsyncDemoFilter in swift example app (#1647)
examples: fix AsyncDemoFilter in swift example app (#1647)

Description: Bugfix. The AsyncDemoFilter would hold headers-only responses indefinitely, leading to a timeout. Encountered this testing error responses with the example apps.
Risk Level: Low
Testing: Local & CI

Signed-off-by: Mike Schore <270e364e708f2af319f244c09ceba52a03dbd980@gmail.com>
envoyproxy/envoy-mobile,envoyproxy/envoy-mobile,envoyproxy/envoy-mobile,envoyproxy/envoy-mobile,envoyproxy/envoy-mobile,envoyproxy/envoy-mobile,envoyproxy/envoy-mobile,envoyproxy/envoy-mobile,envoyproxy/envoy-mobile