This enables host_matching to be set properly by the time the constructor adds
the static route, and enables the static route to be properly associated with
the required host.
Previously, you could only enable host_matching once your app was already
instantiated (e.g. app.url_map.host_matching = True), but at that point
the constructor would have already added the static route without host matching
and an associated host, leaving the static route in a broken state.
Fixes#1559.
00d6e339ec