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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/utils/merge.spec.js
expect(typeof a.foo).toEqual('undefined');
    expect(typeof a.bar).toEqual('undefined');
    expect(typeof b.bar).toEqual('undefined');
    expect(typeof c.foo).toEqual('undefined');
  });
  
  it('should merge properties', function () {
    var a = {foo: 123};
    var b = {bar: 456};
    var c = {foo: 789};
    var d = merge(a, b, c);
    expect(d.foo).toEqual(789);
    expect(d.bar).toEqual(456);
  });
  it('should merge recursively', function () {
    var a = {foo: {bar: 123}};
    var b = {foo: {baz: 456}, bar: {qux: 789}};
    
    expect(merge(a, b)).toEqual({
      foo: {
        bar: 123,
        baz: 456
      },
      bar: {
        qux: 789
      }
    });
  });
});