Currently, if you use axios with async / await, you get a stack trace like this:

```
Request failed with status code 500
    at createError (/Users/rmenezes/code/seedfi/web/node_modules/axios/lib/core/createError.js:16:15)
    at settle (/Users/rmenezes/code/seedfi/web/node_modules/axios/lib/core/settle.js:17:12)
    at IncomingMessage.handleStreamEnd (/Users/rmenezes/code/seedfi/web/node_modules/axios/lib/adapters/http.js:237:11)
    at IncomingMessage.emit (events.js:208:15)
    at IncomingMessage.EventEmitter.emit (domain.js:471:20)
    at endReadableNT (_stream_readable.js:1154:12)
    at processTicksAndRejections (internal/process/task_queues.js:77:11
```

It doesn't show you much about *what* called Axios., which makes debugging quite difficult!

This is because `settle` calls `createError`:
https://github.com/axios/axios/blob/master/lib/core/settle.js#L12-L24

Which creates a new error with a brand new stack trace:
https://github.com/axios/axios/blob/master/lib/core/createError.js#L15-L18

Perhaps we can fix this by creating an error before the request goes out and stitching it to the new error's stack trace?
test/unit/adapters/http.js
server = http.createServer(function (req, res) {
      res.end(req.headers.authorization);
    }).listen(4444, function () {
      var user = 'foo';
      var headers = { Authorization: 'Bearer 1234' };
      axios.get('http://' + user + '@localhost:4444/', { headers: headers }).then(function (res) {
        var base64 = Buffer.from(user + ':', 'utf8').toString('base64');
        assert.equal(res.data, 'Basic ' + base64);
        done();
      }).catch(done);
    });
  });
  it('should support basic auth with a header', function (done) {
    server = http.createServer(function (req, res) {
      res.end(req.headers.authorization);
    }).listen(4444, function () {
      var auth = { username: 'foo', password: 'bar' };
      var headers = { AuThOrIzAtIoN: 'Bearer 1234' }; /
      axios.get('http://localhost:4444/', { auth: auth, headers: headers }).then(function (res) {
        var base64 = Buffer.from('foo:bar', 'utf8').toString('base64');
        assert.equal(res.data, 'Basic ' + base64);
        done();
      }).catch(done);
    });
  });
  it('should provides a default User-Agent header', function (done) {
    server = http.createServer(function (req, res) {
      res.end(req.headers['user-agent']);
    }).listen(4444, function () {
      axios.get('http://localhost:4444/').then(function (res) {