Fix AnvilRecipe returning mutable result.

Before this fix, with the output of a weld in WELDOUT_SLOT and
the same item type in INPUT1_SLOT, picking up the item in WELDOUT
and moving to INPUT1 swapped the two ItemStacks. Applying any anvil
work to the now INPUT1 item stack was actually modifying the
AnvilRecipe.result.
Merge pull request #548 from mabrowning/anvil_recipe_fix

Fix AnvilRecipe returning mutable result.