PlantUML diagrams are not generated for AsciiDoc format files

Summary

PlantUML diagrams are not generated for AsciiDoc format files - which is incorrect behaviour

PlantUML diagrams are genretaed for Markdown or restructured text - which is correct behaviour

Steps to reproduce

Follow manual: https://docs.gitlab.com/ee/administration/integration/plantuml.html

Install and configure PlantUML

Create an asciidoc file in repository and use example from integration manual.

[plantuml, format="png", id="myDiagram", width="200px"]
--
Bob->Alice : hello
Alice -> Bob : Go Away
--

What is the current bug behavior?

This diagram will not be rendered

What is the expected correct behavior?

The diagramm should be rendered

GitLab environment

GitLab 10.8.4-ee (f80a52846a4)
GitLab Shell 7.1.2
GitLab Workhorse v4.2.1
GitLab API v4
Ruby 2.3.7p456
Rails 4.2.10
postgresql