Can't handle setting "now" to 0, returns Date.now() instead

[noyesa]

ember-date-service/addon-test-support/date.js

Line 17 in c72295e

if (this._now) {

This line considers a value of 0 to be falsy, so if you use this it will continue to return the value of Date.now() if you use dateService.setNow(0). This was unexpected to me since it continued to use the real time in my test.