WIP make sure $el contains on the el

Alternative solution to #3421

This is a WIP and needs tests, but I think doing this check once on `_ensureElement` is better than within `attachHtml`  And it makes `this.$el` more consistent and matches what happens in the region constructor

This still needs a test

src/region.js

@@ -133,6 +133,8 @@ const Region = MarionetteObject.extend({
     if (!_.isObject(this.el)) {
       this.$el = this.getEl(this.el);
       this.el = this.$el[0];
+      // Make sure the $el contains only the el
+      this.$el = this.getEl(this.el);
     }
 
     if (!this.$el || this.$el.length === 0) {
@@ -242,7 +244,7 @@ const Region = MarionetteObject.extend({
   // Override this method to change how the new view is appended to the `$el` that the
   // region is managing
   attachHtml(view) {
-    this.Dom.appendContents(this.el, view.el, {_$contents: view.$el});
+    this.Dom.appendContents(this.el, view.el, {_$el: this.$el, _$contents: view.$el});
   },
 
   // Destroy the current view, if there is one. If there is no current view, it does