Fix the class attribute of Droppable widget (#4647)

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>

core/modules/widgets/droppable.js

@@ -39,7 +39,7 @@ DroppableWidget.prototype.render = function(parent,nextSibling) {
 	}
 	// Create element and assign classes
 	var domNode = this.document.createElement(tag),
-		classes = (this["class"] || "").split(" ");
+		classes = (this.droppableClass || "").split(" ");
 	classes.push("tc-droppable");
 	domNode.className = classes.join(" ");
 	// Add event handlers