Fix #334: derivative group by time() fill

NOTE: issue was modified to include only derivative with no group by and derivative with group by + fill.  Issue #1030 was created to fix group by without fill.
This is looks little weird to me, the fact that we took the difference between the `320.0` and `80.0` which are two buckets away (`240000` and `120000`) and emitted no value for the bucket in between `180000`.