**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
lib/defaults.js
}
    }
    return data;
  }],
  /**
   * A timeout in milliseconds to abort a request. If set to 0 (default) a
   * timeout is not created.
   */
  timeout: 0,
  xsrfCookieName: 'XSRF-TOKEN',
  xsrfHeaderName: 'X-XSRF-TOKEN',
  maxContentLength: -1,
  maxBodyLength: -1,
  validateStatus: function validateStatus(status) {
    return status >= 200 && status < 300;
  }
};
defaults.headers = {
  common: {
    'Accept': 'application/json, text/plain, */*'
  }
};
utils.forEach(['delete', 'get', 'head'], function forEachMethodNoData(method) {
  defaults.headers[method] = {};
});
utils.forEach(['post', 'put', 'patch'], function forEachMethodWithData(method) {
  defaults.headers[method] = utils.merge(DEFAULT_CONTENT_TYPE);
});
module.exports = defaults;