Creeper swell value is not stored in NBT

Overview

The swell value of creepers, i.e. how close they are to the explosion, is not written to NBT. This means a creeper will never explode if it is constantly saved and reloaded.
Additionally, this is inconsistent with the maxSwell value (NBT tag Fuse) which is written to NBT.

How to reproduce

1. Summon a creeper
2. Ignite it using flint and steel
3. Just before it explodes close and reopen the world
   • Alternatively sending an ignited creeper through a dimension resets its swell value

Outcome

The creeper does not immediately explode but instead starts to swell again