BeeFileCache serialize崩溃

- (NSData *)serialize:(id)obj
  {
  
  if ( [obj isKindOfClass:[NSData class]] )
      return obj;
  
  if ( [obj respondsToSelector:@selector(JSONData)] ) {
  
  ```
  NSData *data = [obj JSONData];
  return data;
  ```
  
  }
  return nil;
  }
  在iOS9上出现野指针问题
