feat(lucid): allow to unfreeze model instance

[radmen]

I was working on SoftDelete trait for models. I've found that there's a bug in proxyHandler which prevented from unfreezing model.

This PR introduces unfreeze() method which allows to set $frozen to false.

[coveralls]

Coverage increased (+0.003%) to 96.413% when pulling 906c3c0 on radmen:fix-allow-to-change-frozen-attribute into 874268a on adonisjs:develop.

[thetutlage]

Thanks 👍