asciidoctor-maven-plugin: Remote include not working in the Maven plugin

A remote include from GitHub is not working when using the Maven plugin. It’s resolving the include to an empty string.

Here’s an example:

[indent=0]
....
include::https://raw.githubusercontent.com/cometd/cometd/master/pom.xml[tag=jetty-version]
....

This works when using both the Asciidoctor and AsciidoctorJ commandline tools. For some reason, it fails in the Maven plugin. We should include a test for this so we can ensure it doesn’t break once we fix it.