Fix content type assertion

As of werkzeug 1.0 the content-type changed to include "charset=utf-8".

Author: plowman

tests/test_errors.py

@@ -413,7 +413,7 @@ def test_handle_non_api_error(self, app, client):
 
         response = client.get("/foo")
         assert response.status_code == 404
-        assert response.headers['Content-Type'] == 'text/html'
+        assert response.headers['Content-Type'] == 'text/html; charset=utf-8'
 
     def test_non_api_error_404_catchall(self, app, client):
         api = restx.Api(app, catch_all_404s=True)