Fixes #497. Patch Asciidoctor::AbstractBlock#section? in abstract_blo…

[robertpanzer]

…ck.rb to return context by string instead of Symbol because Java blocks always return strings.

There's a problem currently in the way AsciidoctorJ 1.5.x sits on top of Asciidoctor:
It replaces all blocks in the AST once touched by a processor by their Java counterparts, which is necessary.
This makes it impossible for public methods exposed via the Java API to return Ruby specifics like for example Ruby Symbols.

There's a recent change in Asciidoctor 1.5.4 that uses the result of Asciidoctor::AbstractBlock#context and compares it to :section.
As the Java object returns a String instead of a Symbol this test always fails and therefore the error described in asciidoctor/asciidoctor-maven-plugin#258 and filed in #497 occurs.

This PR fixes this by

1. patching abstract_block.rb to compare the String representation of the symbol instead of the symbol
2. Implementing the methods in SectionImpl and AbstractBlockImpl necessary to build a table of contents.

Also tested locally building the asciidoctor-maven-plugin.

[mojavelinux]

Actually, you could almost eliminate any overhead by writing it as:

block.context.to_sym == :section

[mojavelinux]

Very nice explanation! And thanks for making this fix.

[abelsromero]

Since this has been merged and tested with the maven plugin, do you think it makes sense to do a 1.5.4.2 release?
With that we can do a 1.5.4 release of the maven-plugin that passes all tests.

[robertpanzer]

Sounds like a good idea.
I wanted to get asciidoctorj-pdf finished before though.

If we think this is more urgent though, I can also do a release with the old asciidoctorj-pdf.

[abelsromero]

Not urgent, more like something that I should have done before. But still, we can take some time to polish the asciidoctorj-pdf project first.