Allow passing None to url_for() to get the url_prefix

src/calibre/srv/routes.py

@@ -290,5 +290,7 @@ class Router(object):
         return ans
 
     def url_for(self, route, **kwargs):
+        if route is None:
+            return self.url_prefix or '/'
         route = getattr(route, 'route_key', route)
         return self.url_prefix + self.routes[route].url_for(**kwargs)

src/calibre/srv/tests/routes.py

@@ -97,3 +97,4 @@ class TestRouter(BaseTest):
         self.ae(ep, soak_opt), self.ae(args, ['a/b'])
 
         self.ae(router.url_for('/needs quoting', x='a/b c'), '/needs quoting/a%2Fb%20c')
+        self.ae(router.url_for(None), '/')