LP #1103221: pt-heartbeat 2.1.8 doesn't use precision/sub-second timestamps

**Reported in Launchpad by Daniel Nichter last update 15-02-2013 17:11:31

pt-heartbeat 2.1.7 wrote timestamps like,

ts: 2013-01-22T15:40:47.002060

but we broke this in 2.1.8:

ts: 2013-01-22 22:41:08