Fix WSGI signature for DjangoTestAdapter (#7846)

Closes #7132

Author: lovelydinosaur

rest_framework/test.py

@@ -79,7 +79,7 @@ def send(self, request, *args, **kwargs):
             """
             raw_kwargs = {}
 
-            def start_response(wsgi_status, wsgi_headers):
+            def start_response(wsgi_status, wsgi_headers, exc_info=None):
                 status, _, reason = wsgi_status.partition(' ')
                 raw_kwargs['status'] = int(status)
                 raw_kwargs['reason'] = reason