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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/core/mergeConfig.spec.js
expect(merged).not.toBe(defaults);
    expect(merged.headers).not.toBe(defaults.headers);
  });
  it('should allow setting request options', function() {
    var config = {
      url: '__sample url__',
      method: '__sample method__',
      params: '__sample params__',
      data: { foo: true }
    };
    var merged = mergeConfig(defaults, config);
    expect(merged.url).toEqual(config.url);
    expect(merged.method).toEqual(config.method);
    expect(merged.params).toEqual(config.params);
    expect(merged.data).toEqual(config.data);
  });
  it('should not inherit request options', function() {
    var localDefaults = {
      method: '__sample method__',
      data: { foo: true }
    };
    var merged = mergeConfig(localDefaults, {});
    expect(merged.method).toEqual(undefined);
    expect(merged.data).toEqual(undefined);
  });
  ['auth', 'headers', 'params', 'proxy'].forEach(function(key) {
    it('should set new config for' + key + ' without default', function() {
      var a = {}, b = {}, c = {}
      a[key] = undefined
      b[key] = { user: 'foo', pass: 'test' }