def next_feed(feed = nil)
      return Feed.load_xml(get_collection(collection_uri).body) if feed.nil?
      return nil unless feed.has_next?
      Feed.load_xml(get_collection(feed.next_uri).body)
    end
def next_feed(feed = nil)
      return Feed.load_xml(get_collection(collection_uri).body) if feed.nil?
      return nil unless feed.has_next?
      Feed.load_xml(get_collection(feed.next_uri).body)
    end
Get the next feed of the given feed.
 Return the first feed if no argument is passed.
 @param [Hatenablog::Feed] feed blog feed
 @return [Hatenablog::Feed] next blog feed