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**Describe the bug**

Param with value of null is replaced with "{}"

**To Reproduce**

axios.get('/whatever', {params: {test: null}}) used to make GET request to /whatever, now it's /whatever?test=%7B%7D

**Expected behavior**

Should request to /whatever.

**Environment:**
 - Axios Version [the current master]

**Additional context/Screenshots**

See https://github.com/axios/axios/pull/2656#issuecomment-598269460.
test/specs/requests.spec.js
describe('requests', function () {
  beforeEach(function () {
    jasmine.Ajax.install();
  });
  afterEach(function () {
    jasmine.Ajax.uninstall();
  });
  it('should treat single string arg as url', function (done) {
    axios('/foo');
    getAjaxRequest().then(function (request) {
      expect(request.url).toBe('/foo');
      expect(request.method).toBe('GET');
      done();
    });
  });
  it('should treat method value as lowercase string', function (done) {
    axios({
      url: '/foo',
      method: 'POST'
    }).then(function (response) {
      expect(response.config.method).toBe('post');
      done();
    });