As per the subject, when a heading line is included within a
`#+BEGIN_EXPORT backend ... #+END_EXPORT' block, it breaks the parsing
of the block, which is instead split into multiple `paragraph' elements.

See the attached example (test.org), and associate exports (test.tex)
and (test.html).

Tested with `emacs -Q -l minimal-org.el', with the org package installed
from http://orgmode.org/elpa/, org-20161102.