message: add if-modified-since to items.get()
diff: @@ -315,9 +315,10 @@ export class AccountingAPIClient extends BaseAPIClient {
  endpoint = endpoint + '/' + args.Code;
  delete args.Code;
  }
  + const headers = this.generateHeader(args);
  endpoint += generateQueryString(args);
  - return this.oauth1Client.get<ItemsResponse>(endpoint);
  + return this.oauth1Client.get<ItemsResponse>(endpoint, headers);
  },
  create: async (items: Item | Item[], args?: { summarizeErrors?: boolean }): Promise<ItemsResponse> => {
  const endpoint = 'items' + generateQueryString(args, true);
  