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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
expect(reason instanceof Error).toBe(true);
      expect(reason.config.method).toBe('get');
      expect(reason.config.url).toBe('http://thisisnotaserver/foo');
      expect(reason.request).toEqual(jasmine.any(XMLHttpRequest));
      
      jasmine.Ajax.install();
      done();
    };
    axios('http://thisisnotaserver/foo')
      .then(resolveSpy, rejectSpy)
      .then(finish, finish);
  });
  it('should reject on abort', function (done) {
    var resolveSpy = jasmine.createSpy('resolve');
    var rejectSpy = jasmine.createSpy('reject');
    var finish = function () {
      expect(resolveSpy).not.toHaveBeenCalled();
      expect(rejectSpy).toHaveBeenCalled();
      var reason = rejectSpy.calls.first().args[0];
      expect(reason instanceof Error).toBe(true);
      expect(reason.config.method).toBe('get');
      expect(reason.config.url).toBe('/foo');
      expect(reason.request).toEqual(jasmine.any(XMLHttpRequest));
      done();
    };
    axios('/foo')
      .then(resolveSpy, rejectSpy)
      .then(finish, finish);
    getAjaxRequest().then(function (request) {
      request.abort();