Using an anvil or enchanting table let's players make exp bottles even after hitting level 0

[Icohedron]

"The problem is, as stated, that the Minecraft server tracks total experience and levels (and the current fraction of experience between two levels) separately. When changing a player's total experience via the /xp command, Minecraft also ensures that the experience levels are updated. However, this does not happen the other way around, if the levels are updated, the xp is not. This doesn't just happen using the /xp command, you also observe this behaviour when you repair an item using an anvil and enchant an item using the enchantment table." -- dualspiral, Sponge Contributor

[Icohedron]

Also doing /bottle after enchanting or using an anvil will cause the experience to be regained.

[Icohedron]

Merged with issue #2