Added notes to {__proto__} test

Firefox incorrectly allows the __proto__ syntax to affect shorthand
methods and shorthand properties, when in reality it only affects the
PropertyName : AssignmentExpression production.

Also added a check to ensure that __proto__ works with null. The results
were not affected by this.
Merge pull request #223 from webbedspace/null_proto_literals

Added Firefox notes to the {__proto__} test