Fix Python example unary/stream conflict

examples/python/interceptors/headers/generic_client_interceptor.py

@@ -33,14 +33,14 @@ class _GenericClientInterceptor(
                                request):
         new_details, new_request_iterator, postprocess = self._fn(
             client_call_details, iter((request,)), False, True)
-        response_it = continuation(new_details, new_request_iterator)
+        response_it = continuation(new_details, next(new_request_iterator))
         return postprocess(response_it) if postprocess else response_it
 
     def intercept_stream_unary(self, continuation, client_call_details,
                                request_iterator):
         new_details, new_request_iterator, postprocess = self._fn(
             client_call_details, request_iterator, True, False)
-        response = continuation(new_details, next(new_request_iterator))
+        response = continuation(new_details, new_request_iterator)
         return postprocess(response) if postprocess else response
 
     def intercept_stream_stream(self, continuation, client_call_details,