fix lexical comparison of strings in json
Failing test for GiST indexes on jsonb arrays

Even though in some contexts jsonb arrays of strings have as keys (or at
least indexable values) their string elements, this only works with
seqscan plans.  Both GiST and GIN are broken here, which I'm inclined to
consider a submission-blocking issue.

This presumably breaks non-utf8 tests for now, but currently it isn't
clear to me how I ought to deal with that, so pending an answer I'll go
ahead and commit this.