Incrementing leg index removes path traveled and the past leg

Currently we use navigation to pick up someone from a Waypoint and take them to the destination Waypoint. In some cases as the rider has joined before reaching his waypoint i want to mark this pickup waypoint as completed and reroute to destination and we do this by setting:

`navigationService.routeProgress.legIndex += 1`

The problem is this seem to recreate the route with the start point being my location and end point being the destination. What i want is to keep the previous leg and just reroute to destination on a new leg.

MapboxNavigation 1.0

Start with navigationService.routeProgress and the legIndex update described in the issue, using the MapboxNavigation 1.0 pickup-and-destination scenario to reproduce the route reset. Done means marking the pickup waypoint complete preserves the traveled path and past leg while rerouting to the destination on a new leg.

Written by the indexing model from the issue text.