code: public static String normalizeCharset(String s, int offset, int length) {
          String n = CHARSETS.get(s, offset, length);
          return (n == null) ? s.substring(offset, offset + length) : n;
      }
docs: Convert alternate charset names (eg utf8) to normalized name (eg UTF-8).
  
  @param s      the charset to normalize
  @param offset the offset in the charset
  @param length the length of the charset in the input param
  @return the normalized charset (or null if not found)