[O] Bug ignoreheading not respected in lower level headlines latex export

[Charles Millar]

Hi,

Set up -

Org mode version 9.1.13 (release_9.1.13-791-gcecbe4 @ /usr/local/share/org-mode/lisp/)
GNU Emacs 27.0.50 (build 30, x86_64-pc-linux-gnu, GTK+ Version 3.22.11) of 2018-06-02

I group related documents as second level headlines, thus

* All docs :ignoreheading:

** Doc 1 :ignoreheading:

** Doc 2 :ignoreheading:

The first level has an export_file_name property so that I can export all the docs at once; likewise each second level has its own export_file_name for separate export, if necessary.

When I export all the docs from the first level headline the resulting export is

1. Doc 1

content

 (newpage)

2. Doc 2

content

I expect

content

(newpage)

content.

If I export each doc from its second level, the headline is ignored.