Add spec checking that Set#difference doesn't modify original Sets
Add spec checking that Set#exclusion doesn't modify original Sets
Add extra spec checking that Set#filter doesn't modify receiver
Update style of Set#group_by spec, add tests for Set immutability
Update style of Set#intersection spec, add tests that it doesn't modify original Sets
Update style of Set#join spec, add one test for Set immutability
Update style of Set#map spec
Add a test that Set is immutable even when Array used as initializer is mutated
Update style of Set#partition spec, check that it doesn't modify receiver
Add tests to check that Set#sort and #sort_by don't modify receiver
Update style of Set#sum spec, add check that it doesn't modify receiver
Update style of Set#to_a spec, add check that it doesn't modify receiver
Update style of Set#to_list spec, add check that it doesn't modify receiver
Update style of Set#union spec, add check that it doesn't modify original Sets