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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
response = res;
    });
    getAjaxRequest().then(function (request) {
      request.respondWith({
        status: 200,
        statusText: 'OK',
        responseText: '{"foo": "bar"}',
        headers: {
          'Content-Type': 'application/json'
        }
      });
      setTimeout(function () {
        expect(response.data.foo).toEqual('bar');
        expect(response.status).toEqual(200);
        expect(response.statusText).toEqual('OK');
        expect(response.headers['content-type']).toEqual('application/json');
        done();
      }, 100);
    });
  });
  it('should not modify the config url with relative baseURL', function (done) {
    var config;
    axios.get('/foo', {
        baseURL: '/api'
    }).catch(function (error) {
        config = error.config;
    });
    getAjaxRequest().then(function (request) {
      request.respondWith({
        status: 404,