add test for usage of data-embed-code in serialize() function
add test for usage of data-embed-code in serialize() function

orthes/medium-editor-insert-plugin,anorudes/medium-editor-embed-plugin,fedetorre/medium-editor-insert-plugin,fedetorre/medium-editor-insert-plugin,fedetorre/medium-editor-insert-plugin,orthes/medium-editor-insert-plugin