small JSON encoder improvement

in my test I see a suprisingly big performance improvement with this
one:
30s instead of 38s

diff --git a/lib/rfc/json.scm b/lib/rfc/json.scm
index 73fd021ab..48fcd5956 100644
--- a/lib/rfc/json.scm
+++ b/lib/rfc/json.scm
@@ -307,9 +307,9 @@
              (if (>= code #x10000)
                (for-each hexescape (ucs4->utf16 code))
                (hexescape code)))]))
-  (display "\"")
+  (write-char #\")
   (string-for-each print-char str)
-  (display "\""))
+  (write-char #\"))
 
 (define (construct-json x :optional (oport (current-output-port)))
   (with-output-to-port oport