code: public static String getCharset(HttpURLConnection conn) {
  		if (conn == null) {
  			return null;
  		}
  		return ReUtil.get(CHARSET_PATTERN, conn.getContentType(), 1);
  	}
docs: 从Http连接的头信息中获得字符集<br>
  从ContentType中获取
  
  @param conn HTTP连接对象
  @return 字符集