### Describe the bug

When upgrading from `1.2.0-alpha.1` to just `1.2.0` the config `transformResponse` sends data as binary instead of string.

### To Reproduce

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### Code snippet

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### Expected behavior

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### Axios Version

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### Adapter Version

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### Browser

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### Browser Version

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### Node.js Version

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### OS

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### Additional Library Versions

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### Additional context/Screenshots

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test/unit/adapters/http.js
server = app.listen(3001, function () {
        return axios.post('http://localhost:3001/', obj, {
          headers: {
            'content-type': 'application/x-www-form-urlencoded'
          }
        })
          .then(function (res) {
            assert.deepStrictEqual(res.data, obj);
            done();
          }).catch(done);
      });
    });
  });
  it('should respect formSerializer config', function (done) {
    const obj = {
      arr1: ['1', '2', '3'],
      arr2: ['1', ['2'], '3'],
    };
    const form = new URLSearchParams();
    form.append('arr1[0]', '1');
    form.append('arr1[1]', '2');
    form.append('arr1[2]', '3');
    form.append('arr2[0]', '1');
    form.append('arr2[1][0]', '2');
    form.append('arr2[2]', '3');
    server = http.createServer(function (req, res) {
      req.pipe(res);
    }).listen(3001, () => {
      return axios.post('http://localhost:3001/', obj, {
        headers: {