added assert statements into tests/test_requests/test_header_validati…

…on in regards to the issue #6551

tests/test_requests.py

@@ -1702,7 +1702,7 @@ def test_header_validation(self, httpbin):
         }
         r = requests.get(httpbin("get"), headers=valid_headers)
         for key in valid_headers.keys():
-            valid_headers[key] == r.request.headers[key]
+            assert valid_headers[key] == r.request.headers[key]
 
     @pytest.mark.parametrize(
         "invalid_header, key",