Made gold pan accept new gravel blocks.

I also added a null check for the chunk data, as it was turning up null
for me and causing crashes on a couple of my test worlds.
Fix chunk data saving.

For some reason the chunk data tag was being wiped before being saved,
not sure why, probably just a mistake.
I just commented the line in question out, perhaps someone can explain
its purpose.
Merge pull request #558 from simmpole/simmpole-fix

Fix chunk data saving and gold panning gravel.