code: static void addEncoding(final Request.Builder builder, final HttpExchange httpExchange) {
          final String contentType = httpExchange.getRequestHeaders().getFirst("Content-Type");
          if (contentType != null) {
              final Matcher matcher = CHARSET_PATTERN.matcher(contentType);
              if (matcher.matches()) {
                  try {
                      builder.encoding(Charset.forName(matcher.group(1)));
                  }
                  catch (UnsupportedCharsetException e) {
                      //just ignore, fallback encoding will be used instead
                  }
              }
          }
      }
docs: package protected for testing purposes