Fix error on beforeunload in browsers with native support

This commit updates the `beforeunload` event handler to call the
`disable` function instead of `disablePlaceholders` to take advantage
of the condition that checks for native support. Otherwise, in
`handleElem`, the for loop would raise an error because `handleInputs`
would be undefined.
Merge pull request #53 from jonhinson/fix/native_browsers_beforeunload

Fix error on beforeunload in browsers with native support