Add test to ensure _.flatten returns empty arrays for non array-like objects.

test/test.js

@@ -2625,6 +2625,11 @@
 
       deepEqual(_.flatten(array, true), expected);
     });
+
+    test('should return an empty array for non array-like objects', 1, function() {
+      var actual = _.flatten({ 'a': 1 }, _.identity);
+      deepEqual(actual, []);
+    });
   }(1, 2, 3));
 
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