Handle ipv6 addresses in host-header correctly with TLS

When a url uses an ipv6-addresses as the host part, the host-header of the request will be

[::1]:3000

(for ipv6 address ::1). To verify the IP address against a TLS certificate, we need to extract the
IP-address correctly.

Requires node with nodejs/node#14736 resolved to work.

lib/_http_agent.js

@@ -196,7 +196,21 @@ Agent.prototype.addRequest = function addRequest(req, options) {
     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
+      if (hostHeader.startsWith('[')) {
+        const index = hostHeader.indexOf(']');
+        if (index === -1) {
+          // Leading '[', but no ']'. Need to do something...
+          options.serverName = hostHeader;
+        } else {
+          options.servername = hostHeader.substr(1, index - 1);
+        }
+      } else {
+        options.servername = hostHeader.split(':')[0];
+      }
     }
   }