**Describe the bug**
According to [RFC 8259 - The JavaScript Object Notation (JSON) Data Interchange Format](https://tools.ietf.org/html/rfc7159) content-type `application/json` does not have any charset defined, as it must be utf-8 encoded. Older specifications allowed other unicode encodings, and parsers were detecting exact encoding used by first four bytes, still not by charset param.

Axios defaults to header `Content-Type: application/json;charset=utf-8` for some types of sent data.

This breaks some HTTP servers and applications, who are not recognizing the content-type.

**To Reproduce**
```js
axios.post("http://example.com", {an: "object"});
// Content-Type: application/json;charset=utf-8 header will be set
```

**Expected behavior**
`Content-Type: application/json` header should be set as default for object data, without charset parameter.

**Environment:**
 - Axios Version - 0.18.0, I've also checked master branch source - it is still there.
https://github.com/axios/axios/blob/b4c5d35d2875191dfa8c3919d4227dce8e2ad23f/lib/defaults.js#L52

**Additional context/Screenshots**
This breaks some parts of the rabbitmq-management API.

Related issues:
https://github.com/axios/axios/issues/859
test/unit/adapters/http.js
server = http.createServer(function (req, res) {
      res.setHeader('Content-Type', 'application/json;charset=utf-8');
      var bomBuffer = Buffer.from([0xEF, 0xBB, 0xBF])
      var jsonBuffer = Buffer.from(JSON.stringify(data));
      res.end(Buffer.concat([bomBuffer, jsonBuffer]));
    }).listen(4444, function () {
      axios.get('http://localhost:4444/').then(function (res) {
        assert.deepEqual(res.data, data);
        done();
      });
    });
  });
  it('should redirect', function (done) {
    var str = 'test response';
    server = http.createServer(function (req, res) {
      var parsed = url.parse(req.url);
      if (parsed.pathname === '/one') {
        res.setHeader('Location', '/two');
        res.statusCode = 302;
        res.end();
      } else {
        res.end(str);
      }
    }).listen(4444, function () {
      axios.get('http://localhost:4444/one').then(function (res) {
        assert.equal(res.data, str);
        assert.equal(res.request.path, '/two');
        done();
      });
    });