This should do it:
add_action('admin_notices', 'https_the_content');
function https_the_content() {
global $post;
if(!$post->post_content)
return;
//change src to use the current url scheme
$post->post_content = str_replace(array("src=\"http://", "src="http://"), array("src=\"//", "src="https://"), $post->post_content);
return $post->post_content;
}