message: allow if-match header
  because we use it for deletion version safety
diff: @@ -16,7 +16,7 @@ module.exports = function(searcher, writers, optionsArg) {
  ctxt.response.set('Access-Control-Allow-Origin', '*');
  if (ctxt.request.method === 'OPTIONS') {
  ctxt.response.set('Access-Control-Allow-Methods', 'GET,POST,PATCH,DELETE,OPTIONS');
  - ctxt.response.set('Access-Control-Allow-Headers', 'Authorization, Content-Type');
  + ctxt.response.set('Access-Control-Allow-Headers', 'Authorization, Content-Type, If-Match');
  ctxt.status = 200;
  return;
  }
  