Issue 6573: set union method ignores arguments appearing after the original set

When calling the union method of a set with several arguments, if one of
those sets is the original set, all arguments appearing after it are
ignored.  For example:

x = set()
x.union(set([1]), x, set([2]))

evaluates to set([1]), not set([1, 2]) as expected.  As another example,
since all empty frozensets are the same,

frozenset().union(frozenset([1]), frozenset(), frozenset([2]))

also evaluates to just frozenset([1]).

The fix is trivial, so I'm attaching a patch.