**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
var zlib = require('zlib');
var assert = require('assert');
var fs = require('fs');
var path = require('path');
var pkg = require('./../../../package.json');
var server, proxy;
describe('supports http with nodejs', function () {
  afterEach(function () {
    if (server) {
      server.close();
      server = null;
    }
    if (proxy) {
      proxy.close()
      proxy = null;
    }
    if (process.env.http_proxy) {
      delete process.env.http_proxy;
    }
    if (process.env.no_proxy) {
      delete process.env.no_proxy;
    }
  });
  it('should throw an error if the timeout property is not parsable as a number', function (done) {
    server = http.createServer(function (req, res) {
      setTimeout(function () {
        res.end();
      }, 1000);
    }).listen(4444, function () {
      var success = false, failure = false;