look-behind does not take Unicode case-folding into account

[k-takata]

Originally reported in #17 (comment) .

>> /(?:\u{149})a/ui =~ "[\u{2bc}na]"  # Unicode case-folding match is in effect.
=> 1
>> /(?<=\u{149})a/ui =~ "[\u{2bc}na]"  # But look-behind does not take account of it.
=> nil

[k-takata]

Fixed in devel-6.0 branch. Closing.