Fix checkbox creation in numbered lists
* lisp/org.el (org-insert-heading): Check for checkbox at the
beginning of the item, not just at the beginning of the line.
Without this fix, M-RET in numbered lists would create a checkboxed
item because it was using match data from a previous match. Now we
make sure we go back to the beginning of the item, so that the match
really tells us if there was a checkbox there.