Set#delete and Map#delete should return false unless a deletion occur…

…red.

Relates to #298.

Per:
 - b90f0a0#commitcomment-8326164
 - https://people.mozilla.org/%7Ejorendorff/es6-draft.html#sec-set.prototype.delete
 - https://people.mozilla.org/%7Ejorendorff/es6-draft.html#sec-map.prototype.delete

es6-shim.js

@@ -1905,7 +1905,8 @@
           'delete': function (key) {
             var fkey;
             if (this._storage && (fkey = fastkey(key)) !== null) {
-              return delete this._storage[fkey];
+              var hasFKey = _hasOwnProperty.call(this._storage, fkey);
+              return (delete this._storage[fkey]) && hasFKey;
             }
             ensureMap(this);
             return this['[[SetData]]']['delete'](key);

test/collections.js

@@ -202,6 +202,11 @@ describe('Collections', function () {
       expect(function () { return Map.prototype.size; }).to['throw'](TypeError);
     });
 
+    it('should return false when deleting a nonexistent key', function () {
+      expect(map.has('a')).to.equal(false);
+      expect(map.delete('a')).to.equal(false);
+    });
+
     it('should have keys, values and size props', function () {
       map.set('a', 1);
       map.set('b', 2);
@@ -431,6 +436,7 @@ describe('Collections', function () {
       set = new Set();
       testSet = function (key) {
         expect(set.has(key)).to.equal(false);
+        expect(set['delete'](key)).to.equal(false);
         set.add(key);
         expect(set.has(key)).to.equal(true);
         expect(set['delete'](key)).to.equal(true);
@@ -459,6 +465,11 @@ describe('Collections', function () {
       expect(set.add({})).to.equal(set);
     });
 
+    it('should return false when deleting an item not in the set', function () {
+      expect(set.has('a')).to.equal(false);
+      expect(set.delete('a')).to.equal(false);
+    });
+
     it('should accept an iterable as argument', function () {
       testSet('a');
       testSet('b');