code: public void toJson(Writer writer) throws IOException {
  		startHash(writer);
  
  		int size = size();
  		Set<String> set = keySet();
  		int i = 0;
  		for (String key : set) {
  
  			putKey(writer, key);
  			JsonUtil.put(writer, get(key));
  
  			if (i + 1 < size) {
  				addSeparator(writer);
  			}
  			i++;
  		}
  
  		endHash(writer);
  	}
docs: Encodes into the JSON format.
  @param writer {@link Writer} to be used for writing
  @throws IOException
  @author vvakame