Switch wp_dead to plugins_loaded

So right now works only on frontend requests that if you are behind a cache system the filter wp_head is not executed.

It is better to use the plugins_loaded that is executed in any solution including wp-cli, that is executed when all the plugins are loaded.
In this way also behind a cache system it is executed.

Also it is always better to remove the final ?> with the empty line after to not break the html page output in a php file.