Bigtable Date system test broken

After merging #2116, I noticed we had a system error:

https://github.com/GoogleCloudPlatform/google-cloud-node/blob/e353ce0c9010dde1a85edd2610dbca68a5f0619a/packages/bigtable/system-test/bigtable.js#L895

```
Uncaught AssertionError: false == true
      + expected - actual

      -false
      +true
```

@callmehiphop could you ptal?