preserve_host_header option omits port number from the original Host header

[aiotter]

Plug's conn.host is not equal to Host HTTP header.

According to MDN, Host header contains the host and port number of the server to which the request is being sent.
On the other side, conn.host contains only the host.

Even though you enable preserve_host_header option, the Host header is not preserved currently.
The port number is omitted.

This PR is to fix the problem and preserve original Host header intact.

[aiotter]

I don't know why CI is failing. mix test went without error on my laptop.