Custom books opened by non-admin players do not work correctly with selectors.

This problem was noted in multiplayer and can be reproduced in single-player in the following fashion:

1. Go into a survival single-player world with cheats disabled.
2. Open to lan and enable cheats.
3. /give @a minecraft:written_book {title:"",author:"",pages:["[\{\"selector\":\"@p\"\}]"]}
4. do NOT read the book at this stage.
5. Save and Quit to Title
6. Go back into the world (cheats are now disabled again).
7. Read the book given above.
8. Instead of converting the selectors, the raw json text is displayed as the page contents.

Expected behaviour: The book should display the name of the player who opened it regardless of whether cheats are available for the player.

The current behaviour makes the feature somewhat useless for custom map-makers and server utilities.

Testing indicates this bug has been present in some form or other since the feature was introduced in Minecraft 1.8.

Versions 17w43b or earlier, including 1.12.2 result in a blank page rather than raw json text.

Versions 17w45a through 1.13-pre3 of the 1.13 development snapshots worked CORRECTLY!