include statement does not work if it has attributes

[Fethbita]

An example to test
Create basic.adoc an add the following as contents:

= Basic document
:foldername: folder
:filename: folder/test2.adoc

Small test to show that include does not work if attributes are used in the file name:

include::{foldername}/test2.adoc[]
// fails with `asciidoctor: WARNING: dropping line containing reference to missing attribute: foldername`

include::{filename}[]
// fails with `asciidoctor: WARNING: dropping line containing reference to missing attribute: filename`

Create folder folder and test2.adoc within it with any content. Running asciidoctor-latex test.adoc results in:

asciidoctor: WARNING: dropping line containing reference to missing attribute: foldername
asciidoctor: WARNING: dropping line containing reference to missing attribute: filename