Fix flatten init prefix

Author: sergeysova

lib/flatten.js

@@ -2,7 +2,7 @@ function flattenize(routes, prefix, parent) {
   const list = {}
 
   Object.keys(routes.local).forEach(method => {
-    const localName = prefix === '/' ? '/' : `${prefix}/`
+    const localName = prefix === '' ? '/' : `${prefix}/`
 
     if (!list[localName]) {
       list[localName] = {}