Make _.merge assign null values. [closes #151]

Former-commit-id: 5a839996db9475182d5957d2f8cb4b3c265b0d9f

test/test.js

@@ -1217,6 +1217,16 @@
       _.reduce(array, _.merge, actual);
       deepEqual(actual, { 'a': 1, 'b': 2, 'c': 3});
     });
+
+    test('should assign `null` values', function() {
+      var actual = _.merge({ 'a': 1 }, { 'a': null });
+      strictEqual(actual.a, null);
+    });
+
+    test('should not assign `undefined` values', function() {
+      var actual = _.merge({ 'a': 1 }, { 'a': undefined });
+      equal(actual.a, 1);
+    });
   }(1, 2, 3));
 
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