query string should be passed to Flow when using flow.route

[vlukashov]

When navigating to a server-side view with flow.route I want the query string to be available to the server-side navigation event handlers via event.getLocation().getQueryParameters(). With flow-3.0.0.alpha2 this is not the case.

Steps to reproduce:

1. clone the ccdm branch of the beverage buddy starter
2. create a navigation link like <a href="/?foo=bar">Reviews</a> in the main layout
3. implement the BeforeEnterObserver interface in the ReviewsList view to receive BeforeEnterEvent
4. see that in the event handler event.getLocation().getQueryParameters() returns an empty map whereas the expected behavior is that the map contains the foo=bar pair.