Fix a jshint warning

I ignored the warnings about __proto__ because it has been revived in ES6, and the one about a missing 'break' before 'default' because it looks as if the fall-through is intentional.

Five warnings
101	The body of a for in should be wrapped in an if statement to filter unwanted properties from the prototype.
231	The '__proto__' property is deprecated.
241	The '__proto__' property is deprecated.
243	The '__proto__' property is deprecated.
287	Expected a 'break' statement before 'default'.