Dispensers with bucket don't work on cauldrons.

I had an dispenser with a bucket pointed to a cauldron filled with water. I expected the bucket to fill after the dispenser was powered, but the bucket was dropped as an item into the cauldron.

Steps to Reproduce:
1.  fill a cauldron (with water or lava)
2.  have a dispenser containing a bucket pointing to it.
3.  power the dispenser.