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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/headers.spec.js
axios('/foo');
    getAjaxRequest().then(function (request) {
      for (var key in headers) {
        if (headers.hasOwnProperty(key)) {
          expect(request.requestHeaders[key]).toEqual(headers[key]);
        }
      }
      done();
    });
  });
  it('should not set header if value is null', function (done) {
    expect(axios.defaults.headers.common['Accept']).toEqual('application/json, text/plain, */*');
    axios('/foo', {
      headers: {
        Accept: null
      }
    });
    getAjaxRequest().then(function (request) {
      expect(typeof request.requestHeaders['Accept']).toEqual('undefined');
      done();
    });
  });
  it('should add extra headers for post', function (done) {
    var headers = axios.defaults.headers.common;
    axios.post('/foo', 'fizz=buzz');
    getAjaxRequest().then(function (request) {
      for (var key in headers) {
        if (headers.hasOwnProperty(key)) {
          expect(request.requestHeaders[key]).toEqual(headers[key]);
        }