Clocktable: Use inactive time stamps for stepwise table

Clocktable: Use inactive time stamps for stepwise table

Adam Elliott writes:

>  When run in day-step mode, the clocktable header line for each day's
>  table contains an active timestamp.  I figure it should be an inactive
>  timestamp, since otherwise I get a junk entry in the agenda each day
>  (whatever heading was previous to the clocktable).
>
>  I'm talking about the output from a spec such as the following:
>
>  #+BEGIN: clocktable :block thisweek :step day

This commit is the patch Adam sent in.