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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
c[key] = { user: 'foo', pass: 'test' }
      expect(mergeConfig(a, b)).toEqual(c);
    });
    it('should merge ' + key + ' with defaults', function() {
      var a = {}, b = {}, c = {};
      a[key] = { user: 'foo', pass: 'bar' };
      b[key] = { pass: 'test' };
      c[key] = { user: 'foo', pass: 'test' };
      expect(mergeConfig(a, b)).toEqual(c);
    });
    it('should overwrite default ' + key + ' with a non-object value', function() {
      [false, null, 123].forEach(function(value) {
        var a = {}, b = {}, c = {};
        a[key] = { user: 'foo', pass: 'test' };
        b[key] = value;
        c[key] = value;
        expect(mergeConfig(a, b)).toEqual(c);
      });
    });
  });
  it('should allow setting other options', function() {
    var merged = mergeConfig(defaults, { timeout: 123 });
    expect(merged.timeout).toEqual(123);
  });
  it('should allow setting custom options', function() {
    var merged = mergeConfig(defaults, { foo: 'bar' });
    expect(merged.foo).toEqual('bar');
  });
});