Issue Description:

Whenever you upgrade a backpack to a higher level and the backpack has anything inside of it, the resulting backpack will be empty. Is this a bug or an intended feature, because I don't really know.

What happens:

You lose your stuff and can't get it back.

What you expected to happen:

I was expecting the backpack to retain its contents when upgraded.

Steps to reproduce (important):

1. Acquire a leather backpack.
2. Put items in said backpack
3. Upgrade backpack to iron (either storage emphasis or upgrade emphasis)
4. Open backpack
5. Rage and put fist through computer screen.

Affected Versions (Do not use "latest"):

• IronBackpacks: 3.0.0
• Minecraft: 1.12.2
• Forge: 14.23.0.2512
• Other Conflicting Mods (may be irrelevant to your issue): Most likely irrelevant (this was reproduced in a world that had only Iron Backpacks, JEI, Light Level Overlay Reloaded, and Journey Map)

Not yet implemented.