Fix previous URL in two-part forward transitions

I have a transitional route that checks the previous URL to make sure that no parameters changed unexpectedly.  (see https://groups.google.com/forum/?fromgroups=#!topic/derbyjs/xcrJZ8vu_fc)

The problem is that if this transitional route runs on the client as a two-part navigation (meaning that Derby first runs a different route to get the the transition's from state, then runs the transition's forward callback), it incorrectly reports the original previous URL to the transition route.

I fixed it to report the URL of the base route that it just ran.