Docx Reader: Fix hdr auto-id when already auto-id.

If header anchors (bookmarks in a header paragraph) already have an
auto-id, which will happen if they're generated by pandoc, we don't want
to rename it twice, and thus end up with an unnecessary number at the
end. So we add a state value to check if we're in a header. If we are,
we don't rename the bookmark -- wait until we rename it in our header
handling.