**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
http.get(opts, function (res) {
          var body = '';
          res.on('data', function (data) {
            body += data;
          });
          res.on('end', function () {
            response.setHeader('Content-Type', 'text/html; charset=UTF-8');
            response.end(proxyAuth);
          });
        });
      }).listen(4000, function () {
        process.env.http_proxy = 'http://user:pass@localhost:4000/';
        axios.get('http://localhost:4444/').then(function (res) {
          var base64 = Buffer.from('user:pass', 'utf8').toString('base64');
          assert.equal(res.data, 'Basic ' + base64, 'should authenticate to the proxy set by process.env.http_proxy');
          done();
        });
      });
    });
  });
  it('should support proxy auth with header', function (done) {
    server = http.createServer(function (req, res) {
      res.end();
    }).listen(4444, function () {
      proxy = http.createServer(function (request, response) {
        var parsed = url.parse(request.url);
        var opts = {
          host: parsed.hostname,
          port: parsed.port,
          path: parsed.path