HTTP agent breaks with JSON over HTTP/2

Steps to reproduce:

1. Create an HTTP agent check to a JSON file on an HTTP/2 enabled server.
2. Set "Convert to JSON"
3. Test the check.
4. Notice that the JSON result is escaped.

Explanation

It seems Zabbix 5 now supports HTTP/2, at least on my distro (Debian 10). But HTTP/2 comes with headers in lower-case (HTTP headers are not case-sensitive). The poller checks the Content-Type header to see if the result is application/json, but this check is sensitive to casing and as the header is named content-type in HTTP/2 this fails.

Fix should be made by doing a case-insensitive compare on this line: https://github.com/zabbix/zabbix/blob/e916b3be645f8a9a689e923fd1fed4d4816565da/src/zabbix_server/poller/checks_http.c#L171