message: fix openPulldown API if never init pulldown before
diff: @@ -31,6 +31,7 @@ export function pullDownMixin(BScroll) {
  BScroll.prototype.openPullDown = function (config = true) {
  this.options.pullDownRefresh = config
  + this._initPullDown()
  }
  BScroll.prototype.closePullDown = function () {
  