text: Title: Authlib 0.15.x crashing with httpx 0.18.2
  Body: After upgrading to HTTPX 0.18.2, the starlette oauth module (provided by Authlib) started to crash.
  
  When i call the oauth authorize_redirect method, i get the following error: Invalid "auth" argument: <httpx._config.UnsetType object at 0x7fb5ac920310>"}
  
  In HTTPX 0.18.1, everything works like a charm.
  