JSON.stringify should use toString of replacer and not valueOf

test/mjsunit/json-replacer-number-wrapper-tostring.js

+// Copyright 2015 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// http://ecma-international.org/ecma-262/6.0/#sec-json.stringify
+// Step 4.b.iii.5.f.i
+
+var calls = 0;
+
+var num = new Number;
+num.toString = function() {
+  calls++;
+  return '';
+};
+num.valueOf = function() {
+  assertUnreachable();
+};
+
+JSON.stringify('', [num]);
+assertEquals(1, calls);