1.6.0-RC.1: isBlock() always return true

[arafalov]

The documentation does not actually seem to say when it should be true/false, but on my sample document it seems to be always true:

(Style: null, ContentModel: simple, Context: paragraph, Class: BlockImpl, IsBlock: true) (Style: null, ContentModel: compound, Context: dlist, Class: DescriptionListImpl, IsBlock: true) (Style: null, ContentModel: compound, Context: section, Class: SectionImpl, IsBlock: true)

[robertpanzer]

Everything StructuralNode should return true on isBlock(). Only PhraseNodes will return false.

[robertpanzer]

And isBlock() currently throws an Exception when called on a org.asciidoctor.ast.Cell or org.asciidoctor.ast.Column. See github.com/asciidoctor/asciidoctor#2963