defining subdomain constraints in router with various TLD lengths

In the new router, it's possible to define a constraint on the subdomain field, like this:

constraints(:subdomain => 'api') do ... end

However, the subdomain method in request takes an argument that says how long the TLD domain is. Default is 1, so if you have a site like server.com, it works correctly, but for other sites (server.co.uk, localhost) you need to call request.subdomain(2) or request.subdomain(0) respectively.

The problem is that while it's possible to do that in a controller method, it's not possible in the router, which makes the constraints(:subdomain) useless in most cases (unless you use a custom class for constraints). This could be solved by changing the request.subdomain() method so that it takes the TLD length from some kind of global setting which can be set in somewhere in config/ directory.