All selector pseudo superselector logic should work with prefixes

[nex3]

If :foo(X) is a superselector of :foo(Y), where X and Y are selectors, :-pfx-foo(X) should also be a superselector of :-pfx-foo(Y). For the most part LibSass gets this right, but is-superselector(":-pfx-current(a)", ":-pfx-current(a)") currently returns false where it should return true.