Something

        echo "before \n";
        print_r($post);
        $post = str_replace(array("\r\n", "\r", "\n"), "<br />", $post);
        echo "\n\nafter\n";
        print_r($post);

before 
<spoiler>Spoiler text</spoiler>\r\nsdf\r\nsdf\r\nsd\r\nfsdf\r\nsdf\r\ns\r\ndf

after
<spoiler>Spoiler text</spoiler>\r\nsdf\r\nsdf\r\nsd\r\nfsdf\r\nsdf\r\ns\r\ndf