Fixing #169 pull request. I destroyed it.

Currently when redirectTo is a function and the function returns blank, hapi-auth-cookie stills tries to do the redirect instead of behaving like redirectTo: undefined. Fixing this will allow issues like #144 where you might want different behavior on an API call vs the browser.

This change will allow something like following code to redirect or not according to a header set

redirectTo (request) {
  const requestedWith = request.headers['x-requested-with'] && request.headers['x-requested-with'].toLowerCase()
  if (requestedWith && (requestedWith === 'ajax' || requestedWith === 'xmlhttprequest')) {
    return undefined
  } else {
    return '/login'
  }
}