code: protected String endWithSlash( String prefix )
      {
          prefix = StringUtils.defaultIfEmpty( prefix, "/" );
          if ( ! prefix.endsWith( "/" ) )
          {
              prefix = prefix + "/";
          }
          return prefix;
      }
docs: Makes sure the string ends with "/"
  
  @param prefix any string, or null.
  @return the prefix with a "/" at the end, never null.