Fix incorrect serialization of signature environment

Closes #229

typed-racket-lib/typed-racket/env/init-envs.rkt

@@ -127,8 +127,9 @@
               (define ty (force (cdr id/ty)))
               `(cons (quote-syntax ,id) ,(sub ty)))
             m))
+     (define serialized-extends (and extends `(quote-syntax ,extends)))
      `(make-Signature (quote-syntax ,name)
-                      (quote-syntax ,extends)
+                      ,serialized-extends
                       (list ,@(serialize-mapping mapping)))]
     [(arr: dom rng rest drest kws)
      `(make-arr ,(sub dom) ,(sub rng) ,(sub rest) ,(sub drest) ,(sub kws))]

typed-racket-test/succeed/require-signature-all-typed.rkt

@@ -0,0 +1,14 @@
+#lang typed/racket
+
+(module a typed/racket
+  (provide foo^)
+
+  (define-signature foo^
+    ([n : Number])))
+
+(require 'a)
+
+(define-unit foo@
+  (import)
+  (export foo^)
+  (define n 5))