`Set_object_enumerated` doesn't store contents of generators

[andrewnags]

Steps To Reproduce

If you construct Set_object_enumerated directly with a generator, its contents won't be stored
(same with Set_object_enumerated_with_category).

sage: from sage.sets.set import Set_object_enumerated
sage: a = Set_object_enumerated(i for i in range(3))
sage: a
{0, 1, 2}
sage: a
{}

Expected Behavior

This is quite unintuitive, since it differs from the behaviour of Set and Python's set.

sage: s = Set(i for i in range(3))
sage: s
{0, 1, 2}
sage: s
{0, 1, 2}
sage: 
sage: s = set(i for i in range(3))
sage: s
{0, 1, 2}
sage: s
{0, 1, 2}

Actual Behavior

As above.

Additional Information

No response

Environment

• OS: Linux
• Sage Version: 10.6

Checklist

• I have searched the existing issues for a bug report that matches the one I want to file, without success.
• I have read the documentation and troubleshoot guide