_doSearch should handle options as a promise.
_doSearch should handle options as a promise.

`this.get('options')` could return a promise, so `_doSearch` is updated to handle it. Otherwise, `_doSearch` could be triggered and improperly excute `this.filter` on a promise, instead of a promises fulfilled value.
cibernox/ember-power-select,chrisgame/ember-power-select,esbanarango/ember-power-select,chrisgame/ember-power-select,esbanarango/ember-power-select,esbanarango/ember-power-select,cibernox/ember-power-select,Dremora/ember-power-select,cibernox/ember-power-select,cibernox/ember-power-select,Dremora/ember-power-select