Port number in Host request header is forced to be specified

[mandel59]

Code to reproduce:

const http = require("http");
const ProxyAgent = require("proxy-agent");

function main() {
  const agent = new ProxyAgent(null);
  const req = http.request(
    {
      protocol: "http:",
      host: "example.com",
      path: "/",
      agent
    },
    res => {}
  );
  req.end();
  console.log("Host:", req.getHeader("Host"));
}

if (typeof require != "undefined" && require.main == module) {
  main();
}

Expected: The value of Host request header is www.example.com
Actual: The value of Host request header is www.example.com:80

To set Agent.prototype.defaultPort (undocumented) property to 80 solves this issue.