org-add-planning-info: Improve deletion handling

org-add-planning-info: Improve deletion handling

* lisp/org.el (org-add-planning-info): Reset point before each
  planning entry search.  Clean up extra spaces.

* testing/lisp/test-org.el (test-org/add-planning-info): Add tests.

- Reposition save-excursion call to reset point every dolist
  iteration.  Otherwise, the order of the entries to be removed
  matters, and a call like

      (org-add-planning-info nil nil 'scheduled 'deadline)

  on

      DEADLINE: <2015-06-26 Fri> SCHEDULED: <2015-06-26 Fri>

  will fail to remove the deadline entry.

- Delete leading white space even when at the beginning of the line so
  that, when org-adapt-indentation is nil, a space is not inserted
  when removing the leading planning info entry.

  This prevents headings with repeating entries like

      SCHEDULED: <2015-06-26 Fri +1w>

  from being closed and recycled with an extra space in front of
  SCHEDULED.

- Add leading white space to the planning info entry regexp to prevent
  leftover white space when removing entries other than the first
  entry.