op level not updated on server

When changing the op-permission-level in the server.properties file players oped before keep the permission level.

How to reproduce:
1. start new server
2. log in with a client
3. op the player
4. stop server
5. change value of op-permission-level
6. start server
7. rejoin the client
8. see what commands you can execute to see the op level

Expected:
The player have the new permission level

Observed:
The player keep the old permission level

Cause & solution:
In the op.json file the players are stored with a set permission level. This should still be possible because it allows good good permission handeling. To fix this bug it would be possible to set level to true to indicate that a player has op but without set level.