Make ambient values part of 'accepted values' during the constraints part of link generation.

[rynowak]

We accidentally changed this behavior from old MVC. This lets us do some pretty useful stuff with constraints so we should bring it back.

Here's an example (old MVC):

routes.MapRoute(
                name: "Default_slug",
                url: "slug/{controller}/{action}",
                defaults: new { controller = "Home", action = "Index", foo = "bar" },
                constraints: new { foo = new MyConstraint() });

routes.MapRoute(
                name: "Default",
                url: "{controller}/{action}}",
                defaults: new { controller = "Home", action = "Index" },
                constraints: new { foo = new MyConstraint() });

If you navigate to / then the second route will match, and produces the ambient values:
{ controller = "Home", action = "Index" }.

Then inside that action, generate a link to another action Url.Action("About").

The inputs to link generation:
values: { action = "About" }
ambient values: { controller = "Home", action = "Index" }

When constraints are called, they see the 'accepted values:
{ controller = "Home", action = "About" }

Note that the value foo = "bar" is not present, even when running the first route's constraints.

Now repeat, but navigate to /slug to match the first route, producing the ambient values:
{ controller = "Home", action = "Index", foo = "bar" }.

Generate the same link:
values: { action = "About" }
ambient values: { controller = "Home", action = "Index", foo = "bar" }

When constraints are called, they see the 'accepted values:
{ controller = "Home", action = "About", foo = "bar" }

Note that foo = "bar" is present because it's an ambient value and not because it's a default value of the route.

[rynowak]

@Eilon - confirmed the the behavior we want is the same as legacy

[rynowak]

Fixed in: 63dcdd6