navigate: Fallback to currentRequest.namedParams

If navigate is called without namedParams, attempt to use the
namedParams of the current request instead.

This will allow anchors like this:
`<a class='navigate' href='/foo/:named/bar'>Click me</a>`
Merge pull request #60 from swipely/fallback_to_current_request

navigate: Fallback to currentRequest.namedParams