Hi,

I'm using axios v0.17.0 with HEAD method like this:

`axios.head('http://example.com'); `

(let's suppose http://example.com is set to redirect 301 to http://www.example.com)

For some reason Request method will change on the first redirect to GET instead of keeping HEAD all the way down. This will read in logs like this:

```
example.com:80 10.0.0.1 - - "HEAD / HTTP/1.1" 301 237 "-" "axios/0.17.0"
example.com:80 10.0.0.1 - - "GET / HTTP/1.1" 200 246 "-" "axios/0.17.0"
```

instead of this:

```
example.com:80 10.0.0.1 - - "HEAD / HTTP/1.1" 301 237 "-" "axios/0.17.0"
example.com:80 10.0.0.1 - - "HEAD / HTTP/1.1" 200 246 "-" "axios/0.17.0"
```

When I do the test in Firefox it keeps original request method (HEAD) all the way down.

Any idea why is this?

Thanks,
Milos

test/unit/adapters/http.js
});
  });
  it('should support UTF8', function (done) {
    var str = Array(100000).join('ж');
    server = http.createServer(function (req, res) {
      res.setHeader('Content-Type', 'text/html; charset=UTF-8');
      res.end(str);
    }).listen(4444, function () {
      axios.get('http://localhost:4444/').then(function (res) {
        assert.equal(res.data, str);
        done();
      });
    });
  });
  it('should support basic auth', function (done) {
    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();
      });
    });
  });
  it('should support basic auth with a header', function (done) {
    server = http.createServer(function (req, res) {
      res.end(req.headers.authorization);