ensure define_webjump's alternate url has root path

ensure define_webjump's alternate url has root path

This is to avoid the extra content-negotiation that results from
requesting an url with no path.

compute_url_pre_path replaced with url_path_trim.

This change causes the following test to pass:

  define_webjump("test1", "http://www.example.com/");
  assert_equals(webjumps.get("test1").handler(),
                "http://www.example.com/");