Fix classNames when no tagName is specified.

Author: Madeorsk

src/trix/views/piece_view.coffee

@@ -70,12 +70,13 @@ class Trix.PieceView extends Trix.ObjectView
       if config.style
         styles[key] = value for key, value of config.style
 
-      if config.className
-        pendingElement.className = config.className
-
     if Object.keys(styles).length
       element ?= makeElement("span")
       element.style[key] = value for key, value of styles
+
+    if config.className
+      element.className = config.className
+
     element
 
   createContainerElement: ->