Otherwise we get undesirable results, as the format's native
citation mechanism is used instead of (or in addition to) the
citeproc-generated citations. Closes#10662.
We used to insert extra spaces to ensure that the content respected
the four-space rule. That is not really necessary now, since pandoc's
markdown and most markdowns don't follow the four-space rule.
Those who want the old behavior can obtain it by using
`-t markdown+four_space_rule`.
Closes#7172.
John MacFarlane