jsrouting: clear search boxes on page unload

This PR fixes the following little gotcha I noticed:

1. Use the routing search (i.e. click "Get directions" then fill in the "From" and "To" and press Go)
1. Navigate away to some other webpage
1. Navigate back to the homepage.
1. Click "Get directions" again. The previously-searched values are still there. Now if you press "Go" (or even if you modify just one of the two search terms) nothing happens, which is awkward.