为什么我的HTTP请求标头字段中缺少主机?

I'm printing out all the headers and I get:

map[Cookie:[_ga=GA1.2.843429125.1462575405] User-Agent:[Mozilla/5.0
(Macintosh; Intel Mac OS X 10_11_3) AppleWebKit/601.4.4 (KHTML, like Gecko)
Version/9.0.3 Safari/601.4.4] Accept-Language:[en-us] 
Accept-Encoding:[gzip, deflate] Connection:[keep-alive] 
Accept:[text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8]]

which means my browser is sending "Cookie", "User-Agent", "Accept-Language", "Accept-Encoding", "Connection", and "Accept" but there is no "Host" value.

How can I get my https://en.wikipedia.org/wiki/Virtual_hosting working without this value?

I'm using https://github.com/gin-gonic/gin

It stated on Golang http docs :

For incoming requests, the Host header is promoted to the Request.Host field and removed from the Header map.

So you can get the host by access

http.Request.Host

Check here for details : https://golang.org/pkg/net/http/