Even though we passed the `class` attribute, it did not take it into
account. For example try to render this:
<$droppable class="custom-css-class">
... content ...
</$droppable>
Before applying this patch:
<span class=" tc-droppable">
... content ...
</span>
After applying:
<span class="custom-css-class tc-droppable">
... content ...
</span>
9c22537b4e