**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/specs/headers.spec.js
testHeaderValue(request.requestHeaders, 'x-header-c', 'c');
      done();
    });
  });
  it('should use application/json when posting an object', function (done) {
    axios.post('/foo/bar', {
      firstName: 'foo',
      lastName: 'bar'
    });
    getAjaxRequest().then(function (request) {
      testHeaderValue(request.requestHeaders, 'Content-Type', 'application/json;charset=utf-8');
      done();
    });
  });
  it('should remove content-type if data is empty', function (done) {
    axios.post('/foo');
    getAjaxRequest().then(function (request) {
      testHeaderValue(request.requestHeaders, 'Content-Type', undefined);
      done();
    });
  });
  it('should preserve content-type if data is false', function (done) {
    axios.post('/foo', false);
    getAjaxRequest().then(function (request) {
      testHeaderValue(request.requestHeaders, 'Content-Type', 'application/x-www-form-urlencoded');
      done();
    });
  });
});