Fix crash if AsciiDoc attributes key is defined as String in playbook file

All the code is present to parse the AsciiDoc attributes from a String, but due to a bug, they cannot be specified this way in the playbook file (only using the CLI). It leads to a crash.

TypeError: Cannot read property 'has' of undefined

It's important to fix this bug because this is a cryptic error.

Here's an example of what would be permitted.

asciidoc:
  attributes: experimental,idprefix,idseparator=-,page-pagination

This matches the syntax accepted by the --attribute option (though the attributes can also be specified individually using multiple --attribute options).