Perform anvil calculations when using 1.5 drop feature.

When using the new feature in 1.5 to drop the item in any highlighted slot,
the anvil result slot does not apply the full anvil calculation that picking
up the item does, including the experience calculation.

Remapped by Srg2Source from Bukkit/CraftBukkit@e4f2dd71526f40223f4fc5b20ecd29986dce9acc