code: String convertCase(String str) {
          if (str == null) {
              return null;
          }
          return isSensitive ? str : str.toLowerCase();
      }
docs: Converts the case of the input String to a standard format.
  Subsequent operations can then use standard String methods.
  
  @param str  the string to convert, null returns null
  @return the lower-case version if case-insensitive