data-fragment-index are not correctly rendered on code blocks

[jtama]

Hi,
I want to synchronized step-by-step syntax highlighting and fragments block on a slide.

For this I used the following :

[.fragment, java, highlight="1|3|5", step=0]
----
Optional<String> foo();

Optional<String> bar();

Optional<String> foobar();
----

[.fragment,data-fragment-index=0]
Foo
[.fragment,data-fragment-index=1]
Bar
[.fragment,data-fragment-index=2]
Foobar

But that doesn't work because of how code block are rendered (the attribute is assigned on the div .listingblock element.

I have a workaround for this and will make a PR.