Unable to add multiple class names to hr element

[tlylt]

Tell us about your environment
Windows 10

• MarkBind Version:
  3.0.6
  What did you do? Please include the actual source code causing the issue.
  According to the User Guide:
  

I should be able to add multiple class names to the block-level element HR

What did you expect to happen?
The --- {.border-danger .bg-light} should result in <hr class="border-danger bg-light">

What actually happened? Please include the actual, raw output.
<hr class="border-danger">

Reproduced at #1675