Only concat concatable scripts and styles

[freibergergarcia]

Expected Behavior

Only scripts and assets that are concatable should be allowed to be overridden by the css_do_concat and js_do_concat filters.

Actual Behavior

Both filters don't take into consideration whether a script/asset is concatable. One example is:
https://github.com/Automattic/nginx-http-concat/blob/master/jsconcat.php#L100-L103

Where you could attempt to concat an external script.

Steps to Reproduce

The simplest way to reproduce is adding:

add_filter( 'js_do_concat', '__return_true' );
// or
add_filter( 'css_do_concat', '__return_true' );