routing: Fix bugs with matching URLs with correct route but missing metadata. When matching a route like `/foo/%slug%` against an URL like `/foo`, the route will (correctly) return a match, but it will be completely missing the `slug` metadata, resulting in problems elsewhere. This change makes it so that any missing route metadata will be filled in with an empty string. And because this means generated URLs may differ from the incoming URL when using trailing slashes (`/foo/` _vs._ `/foo`), we make the assert in the chef server handle those discrepancies.

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