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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
statusText: 'NOT FOUND',
        responseText: 'Resource not found'
      });
      setTimeout(function () {
        expect(config.baseURL).toEqual('/api');
        expect(config.url).toEqual('/foo');
        done();
      }, 100);
    });
  });
  it('should allow overriding Content-Type header case-insensitive', function (done) {
    var response;
    var contentType = 'application/vnd.myapp.type+json';
    axios.post('/foo', { prop: 'value' }, {
      headers: {
        'content-type': contentType
      }
    }).then(function (res) {
      response = res;
    });
    getAjaxRequest().then(function (request) {
      expect(request.requestHeaders['Content-Type']).toEqual(contentType);
      done();
    });
  });
  it('should support binary data as array buffer', function (done) {
    var input = new Int8Array(2);
    input[0] = 1;
    input[1] = 2;
    axios.post('/foo', input.buffer);