Fix overzealous timeseries zero-filling.

When the index's maxTime is not aligned with the query granularity, gran.next
can cause an time extra bucket to get zero-filled. Truncating first prevents
that.
Test for desired timeseries zero-filling behavior.
Merge pull request #705 from metamx/fix-timeseries-zero-filling

Fix extraneous timeseries zero filling at end of data interval