update tests

- Use mockbin for redirections.
- Mock synchronous requests too.

tests/integration/cassettes/DoSyncRequesttest_httpx_test_test_behind_proxy.yml

@@ -1,43 +0,0 @@
-interactions:
-- request:
-    body: null
-    headers:
-      accept:
-      - '*/*'
-      accept-encoding:
-      - gzip, deflate, br
-      connection:
-      - keep-alive
-      host:
-      - httpbin.org
-      user-agent:
-      - python-httpx/0.12.1
-    method: GET
-    uri: https://localhost:8080/headers
-  response:
-    body:
-      string: "{\n  \"headers\": {\n    \"Accept\": \"*/*\", \n    \"Accept-Encoding\"\
-        : \"gzip, deflate, br\", \n    \"Host\": \"httpbin.org\", \n    \"User-Agent\"\
-        : \"python-httpx/0.12.1\", \n    \"X-Amzn-Trace-Id\": \"Root=1-5ea778ca-c402f3ae901e78b3435b0a0c\"\
-        \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 00:28:58 GMT
-      Server:
-      - gunicorn/19.9.0
-      Via:
-      - my_own_proxy
-    status:
-      code: 200
-      message: OK
-version: 1