### 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
};
        https.get(opts, function (res) {
          var body = '';
          res.on('data', function (data) {
            body += data;
          });
          res.on('end', function () {
            response.setHeader('Content-Type', 'text/html; charset=UTF-8');
            response.end(body + '6789');
          });
        });
      }).listen(4000, function () {
        process.env.https_proxy = 'https://localhost:4000/';
        axios.get('https://localhost:4444/', {
          httpsAgent: new https.Agent({
            rejectUnauthorized: false
          })
        }).then(function (res) {
          assert.equal(res.data, '123456789', 'should pass through proxy');
          done();
        }).catch(done);
      });
    });
  });
  it('should re-evaluate proxy on redirect when proxy set via env var', function (done) {
    process.env.http_proxy = 'http://localhost:4000'
    process.env.no_proxy = 'localhost:4000'
    var proxyUseCount = 0;
    server = http.createServer(function (req, res) {
      res.setHeader('Location', 'http://localhost:4000/redirected');