unevaluated ImageSet not recognized as subset of Integers

[smichr]

>>> ImageSet(Lambda(x, x - 3), S.Integers).is_subset(S.Integers)  -> unevaluated
False
>>> imageset(Lambda(x, x - 3), S.Integers)  -> evaluated
Integers

[gschintgen]

With current master is_subset no longer returns False, but None.
So either this can be closed as fixed or doit() could be added to the is_subset handler. It does seem reasonable to me to have subset checks (and also other set operations) operate on evaluated ImageSets.

(Note: there's currently an XFAIL test for this issue.)