Skip already processed prefix characters

Author: akolov

Foundation/NSString.swift

@@ -651,7 +651,7 @@ extension NSString {
         }
 
         if start < end {
-            for char in characters.reverse() {
+            for char in characters.suffixFrom(start).reverse() {
                 guard set.characterIsMember(char) else {
                     break
                 }