tls: handle ipv6 addresses in host-header

When an http-client requests a url with the hostname
specified as an IPv6 address, the Host: header will
have the following format:

Host: [::1]:3000

The servername in this case should be '::1'.

Fixes: nodejs#14736

lib/_http_agent.js

@@ -152,7 +152,12 @@ Agent.prototype.addRequest = function addRequest(req, options, port/*legacy*/,
     options.servername = options.host;
     const hostHeader = req.getHeader('host');
     if (hostHeader) {
-      options.servername = hostHeader.replace(/:.*$/, '');
+      // abc => abc
+      // abc:123 => abc
+      // [::1] => ::1
+      // [::1]:123 => ::1
+      options.servername = hostHeader.replace(/:[^\]]+$/, '')
+        .replace(/^\[(.*)\]$/, '$1');
     }
   }