fix macro for new syntax form

Racket's syntax form now interprets ??, so that identifier
should be quoted (or escaped) so that it gets interpreted as
part of the inner template.

Author: rmculpepper

json-util.rkt

@@ -11,17 +11,18 @@
      (with-syntax ([(key_ ...) (generate-temporaries #'(key ...))]
                    [(keyword ...)
                     (for/list ([k (syntax->datum #'(key ...))])
-                      (string->keyword (symbol->string k)))])
+                      (string->keyword (symbol->string k)))]
+                   [??-id (quote-syntax ??)])
        (syntax/loc stx
          (define-match-expander name
            (λ (stx)
              (syntax-parse stx
                [(_ (~optional (~seq keyword key_)) ...)
-                (quasitemplate/loc stx (hash-table (?? ['key (? ctc key_)]) ...))]))
+                (quasitemplate/loc stx (hash-table (??-id ['key (? ctc key_)]) ...))]))
            (λ (stx)
              (syntax-parse stx
                [(_ (~optional (~seq keyword key_)) ...)
                 (syntax/loc stx
                   (make-hasheq (list (cons 'key key_) ...)))])))))]))
 
-(provide define-json-expander)
+(provide define-json-expander)