file/zip: recognize string ports as seekable

racket/collects/file/zip.rkt

@@ -72,10 +72,11 @@
 
   ;; seekable-port? : port -> boolean
   (define (seekable-port? port)
+    (or (string-port? port)
         (and (file-stream-port? port)
              (with-handlers ([void (lambda (exn) #f)])
                (file-position port (file-position port))
-           #t)))
+               #t))))
 
   (define (write-int n size)
     (write-bytes (integer->integer-bytes n size #f #f)))