[O] blank todo kw does not delete closed ts

[Samuel Wales]

Changing a doneified entry to blank todo state leaves a
closed ts.

#+BEGIN_SRC org
  ,* DONE doneify
  CLOSED: [2013-04-09 Tue 14:11]
  ,* now change to blank todo kw -- notice the closed ts
  CLOSED: [2013-04-09 Tue 14:11]
#+END_SRC

Perhaps it would be more intuitive for newcomers, and safer
for expiry searches like "+CLOSED<\"<-1m>\"" where not
everybody thinks to try to match all done todo keywords (not
even sure what the convention is for that), to remove the
ts.

Thanks.

Samuel

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