Restore Rack::Server#middleware backward compatibility

Fixes regression in #706. It intends to retain backward compatibility with calls to `#middleware` by aliasing the method to the new `default_middleware_for_environment`, but since it doesn't call `#middleware` internally, subclasses that overrode it no longer work.

Restored compatibility by calling `#middleware` internally and delegating its implementation to the new `default_middleware_for_environment` methods.