The code for shearing a pumpkin used minetest.swap_node() to replace a
faceless pumpkin with a carved pumpkin. This did not trigger the node
callbacks of the carved pumpkin, which meant that shearing a pumpkin
would not check for the snow golem or iron golem spawn conditions.
This patch replaces minetest.swap_node() in the code for shearing a
pumpkin with minetest.set_node(), which does trigger the callbacks;
therefore snow and iron golems can now spawn as a pumpkin is carved.
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