code: function(option) {
    option = objectAssign({operation: 'searchRetrieve'}, option);
    this.url = sruURL + '?' + queryString.stringify(option) + '&query=';
  }
docs: SRU(Search/Retrieve Via URL) interface.
  @param {object} option - The Search Option.(see also, http://iss.ndl.go.jp/information/wp-content/uploads/2014/04/ndlsearch_api_all_20140331_en.pdf)
  @constructor