[FIX] web_editor: make sure html editor does not ignore changes

For some reason, Summernote does not trigger a change event in some
cases.  It looks like it happens in 'codeview' mode, and when the change
in question is only a backspace key event.  Maybe it listen for keypress
and backspace do not trigger keypress event.

In any cases, the situation is that it is not easy to clear an html
field: going in edit mode, then selecting the content and pressing
backspace will not work before this commit.

With this commit, we specifically compare the value in the editor with
the current value to determine if the field is dirty.

addons/web_editor/static/src/js/backend.js

@@ -43,6 +43,9 @@ var FieldTextHtmlSimple = basic_fields.DebouncedField.extend(TranslatableFieldMi
             var layoutInfo = this.$textarea.data('layoutInfo');
             $.summernote.pluginEvents.codeview(undefined, undefined, layoutInfo, false);
         }
+        if (this._getValue() !== this.value) {
+            this._isDirty = true;
+        }
         this._super.apply(this, arguments);
     },
     /**