Unable to sign in alfter sign up

### Bug description

WHen i integrate nativeauthenticator and jupyterhub,and jupyterhub's version is 1.1.0,nativeauthenticator 's version is lastest,i can create a new admin user from sign up page,but can not sign in.Error msg is ```Invalid username or password```.


And i rebuild the docker image by using Dockerfile:
FROM jupyterhub/k8s-hub:0.9.1
RUN pip install git+https://github.com/jupyterhub/nativeauthenticator.git

Any suggestions would be greatly appreciated！

#### Expected behaviour

success to sign in
#### Actual behaviour

### How to reproduce

### Your personal set up

Start by reproducing the sign-up and sign-in flow with JupyterHub 1.1.0, the latest nativeauthenticator, and the Dockerfile based on jupyterhub/k8s-hub:0.9.1. Compare the created admin user's state with the authentication failure, and consider the issue done when a newly created admin user can sign in successfully.

Written by the indexing model from the issue text.