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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
function testHeaderValue(headers, key, val) {
  var found = false;
  for (var k in headers) {
    if (k.toLowerCase() === key.toLowerCase()) {
      found = true;
      expect(headers[k]).toEqual(val);
      break;
    }
  }
  if (!found) {
    if (typeof val === 'undefined') {
      expect(headers.hasOwnProperty(key)).toEqual(false);
    } else {
      throw new Error(key + ' was not found in headers');
    }
  }
}
describe('headers', function () {
  beforeEach(function () {
    jasmine.Ajax.install();
  });
  afterEach(function () {
    jasmine.Ajax.uninstall();
  });
  it('should default common headers', function (done) {
    var headers = axios.defaults.headers.common;