making sure to return function object

discussed in https://github.com/zurb/foundation/pull/2239

I had the similar issue as it's described above.  When I ran integration tests with Capybara::Poltergeist driver, I got the error on 

https://github.com/zurb/foundation/blob/master/js/foundation/foundation.js#L186

The reason why Poltergeist raising the error was because inside `init_lib()`, anonymous function inside the first argument of `this.trap`, which eventually chain-call `bind()` sometimes return undefined for some cases.  So I made sure it always returns function object.