Extract cookies to client on every response

This is required because previous extraction code is now patched out by vcpy.

Also handle headers with same key in responses.

tests/integration/cassettes/DoAsyncRequesttest_httpx_test_test_behind_proxy.yml

@@ -20,14 +20,22 @@ interactions:
       : \"python-httpx/0.12.1\", \n    \"X-Amzn-Trace-Id\": \"Root=1-5ea778c9-ea76170da792abdbf7614067\"\
       \n  }\n}\n"
     headers:
-      access-control-allow-credentials: 'true'
-      access-control-allow-origin: '*'
-      connection: keep-alive
-      content-length: '226'
-      content-type: application/json
-      date: Tue, 28 Apr 2020 00:28:57 GMT
-      server: gunicorn/19.9.0
-      via: my_own_proxy
+      access-control-allow-credentials:
+      - 'true'
+      access-control-allow-origin:
+      - '*'
+      connection:
+      - keep-alive
+      content-length:
+      - '226'
+      content-type:
+      - application/json
+      date:
+      - Tue, 28 Apr 2020 00:28:57 GMT
+      server:
+      - gunicorn/19.9.0
+      via:
+      - my_own_proxy
     http_version: HTTP/1.1
     status_code: 200
 version: 1