fix extract_nav_url so nav scroll is correctly preserved (bug 986718)

I noticed looking at the scroll perf - that scroll restoration isn't working pressing back after going to reviews from a details page.

Further investigation showed that the `extract_nav_url` function erroneously returns urls like `/foo/bar?` when no whitelisted url params are present. This results in an extra nav stack entry which then has no scrollTop property hence pressing back cannot restore the scroll.