Test Symbol.toStringTag overrides on primitive wrapper prototypes. Closes gh-809 (#837)

Signed-off-by: Rick Waldron <waldron.rick@gmail.com>

test/built-ins/Object/prototype/toString/symbol-tag-override-primitives.js

+// Copyright (C) 2017 the V8 project authors. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+description: >
+    String values of `Symbol.toStringTag` property override built-in tags
+es6id: 19.1.3.6
+info: >
+    1. If the this value is undefined, return "[object Undefined]".
+    2. If the this value is null, return "[object Null]".
+
+    14. Else, let builtinTag be "Object".
+    15. Let tag be ? Get(O, @@toStringTag).
+    16. If Type(tag) is not String, let tag be builtinTag.
+    17. Return the String that is the result of concatenating "[object ", tag, and "]".
+
+    4.3.2 primitive value
+
+    member of one of the types Undefined, Null, Boolean, Number, Symbol, or String as defined in clause 6
+
+features: [Symbol.toStringTag]
+---*/
+
+
+Boolean.prototype[Symbol.toStringTag] = 'test262';
+assert.sameValue(Object.prototype.toString.call(Boolean.prototype), '[object test262]');
+assert.sameValue(Object.prototype.toString.call(true), '[object test262]');
+
+Number.prototype[Symbol.toStringTag] = 'test262';
+assert.sameValue(Object.prototype.toString.call(Number.prototype), '[object test262]');
+assert.sameValue(Object.prototype.toString.call(0), '[object test262]');
+
+String.prototype[Symbol.toStringTag] = 'test262';
+assert.sameValue(Object.prototype.toString.call(String.prototype), '[object test262]');
+assert.sameValue(Object.prototype.toString.call(''), '[object test262]');
+
+
+Object.defineProperty(Symbol.prototype, Symbol.toStringTag, {
+  value: 'test262'
+});
+
+assert.sameValue(Object.prototype.toString.call(Symbol.prototype), '[object test262]');