[composer] Fix expressions inserted into HTML item source are lost on refresh

Author: nyalldawson

src/app/composer/qgscomposerhtmlwidget.cpp

@@ -377,6 +377,7 @@ void QgsComposerHtmlWidget::on_mInsertExpressionButton_clicked()
       {
         mHtmlEditor->insertAt( "[%" + expression + "%]", line, index );
       }
+      mHtml->setHtml( mHtmlEditor->text() );
       composition->endMultiFrameCommand();
       blockSignals( false );
     }