In my plesk, i have only FastCGI application server by Apache and FPM application server by nginx for run PHP.
My domain was configuered with FastCGI (phpinfo):
Server API: CGI/FastCGI
But the HTTP HEAD return server nginx:
HTTP/1.1 200 OK
Server: nginx
Date: Tue, 18 Apr 2017 20:19:19 GMT...