Superagent fail on 4xx with content-type:"text/plain" and a body

superagent.test.js

// Related tp: https://github.com/visionmedia/superagent/issues/1263

const agent = require("superagent");

const express = require('express')
const app = express()
app.get('/', function (req, res) {
  res.status(400).end("not good");
})
var mockPort;

var callCounter = 0;

function makeACall(headers, body) {
  var _callCounter = ++callCounter;
  var request = agent('GET', "http://127.0.0.1:" + mockPort + "/");
  request.set(headers);
  request.send(body);
  console.log(_callCounter, ') superagent calling', JSON.stringify(headers), JSON.stringify(body));

  request.then(function (res) {
    console.log(_callCounter, ') superagent body ->', res.body);
  })
  .catch(function (err){
    console.error(_callCounter, ') superagent catch -> status ==>', err.status);
    console.error(err.stack || err);
    console.error();
  });
}

// port 0 let operation system choose port
const listener = app.listen(0, function () {

    mockPort = listener.address().port;
    makeACall({"content-type":"text/plain"});
    makeACall({"content-type":"text/plain"}, {foo: 1});
    makeACall({});
    makeACall({}, {foo: 1});
});