Fix ⊊, ⊋ on non-Set arguments

[cmcaine]

These have been broken since at least 84da967 (2018), but probably forever (I didn't follow the blame past the renaming of the file).

Here's an example of getting the wrong answer:

julia> issetequal((1, 2), (1, 2, 2)) # correct
true

julia> issubset((1, 2), (1, 2, 2)) # correct
true

julia> (1, 2) ⊊ (1, 2, 2) # incorrect
true

[cmcaine]

CI failure looks unrelated.