code: @Override
      public FastStr toUpperCase(Locale locale) {
          String s = toString();
          return unsafeOf(s.toUpperCase(locale));
      }
docs: Wrapper of {@link String#toUpperCase(java.util.Locale)} but return FastStr type instance
  
  @param locale the locale
  @return a FastStr instance with all characters from this FastStr
  be converted into uppercase based on the locale specified