code: private static boolean isSupportedCharset(MediaType mediaType) {
          Map<String, String> parameters = mediaType.getParameters();
          if (parameters == null || parameters.isEmpty()) {
              return true;
          }
          String charset = parameters.get("charset");
          return charset == null
                  || "UTF-8".equalsIgnoreCase(charset)
                  || "ISO-8859-1".equalsIgnoreCase(charset);
      }
docs: As content is cached, we expect both ends use UTF-8 always. If no content charset encoding is explicitly
  defined, UTF-8 is assumed as a default.
  As legacy clients may use ISO 8859-1 we accept it as well, although result may be unspecified if
  characters out of ASCII 0-127 range are used.