Issue 471: Fix panic when removing a route that doesn't exist in w.routes.

[jeffreydwalter]

This PR fixes the remaining panic in RemoveRoute().

[jeffreydwalter]

@emicklei I made the changes you suggested. Would like to get this PR merged so I can vendor it back into my codebase. Please let me know if there's anything else I can do to facilitate. Thanks!

[jeffreydwalter]

@emicklei thinking about this further, what do you think about setting the capacity of newRoutes, like this: newRoutes := make([]Route, 0, len(w.routes))? This would potentially allocate an extra item in the slice in the case the route is found and removed, but would cut down on slice allocations. If you think that is better, I'm happy to make another PR.

[jeffreydwalter]

@emicklei what do you say in regard to my last comment? ☝🏻

[emicklei]

@jeffreydwalter , removing a Route is not on the hot-path so any optimisations here are not significant.