additional comma and addAll: test for overlapping Sets
ifAbsentPut: tests

Array does not have at:ifAbsentPut:, ???
Push at:ifAbsentPut: up
Merge branch 'upstream' into at-ifabsentput-tests
Update API-CHANGES.txt
Merge pull request #785 from herby/at-ifabsentput-tests

at:ifAbsentPut: tests, additional tests for Set