Fix included: "Optional" option is not respected for include

[suddenfall]

This adoc code produces an error if the file does not exist:

include::my-dummy.adoc[opts=optional]

But it shouldn't because of the option "optional".

Here is a diff to patch the bug: diff.txt

[suddenfall]

Pull request added: https://github.com/asciidocfx/AsciidocFX/pull/634