### 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
proxy = http.createServer(function (request, response) {
        var parsed = url.parse(request.url);
        var opts = {
          host: parsed.hostname,
          port: parsed.port,
          path: parsed.path
        };
        var proxyAuth = request.headers['proxy-authorization'];
        http.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(proxyAuth);
          });
        });
      }).listen(4000, function () {
        axios.get('http://localhost:4444/', {
          proxy: {
            host: 'localhost',
            port: 4000,
            auth: {
              username: 'user',
              password: 'pass'
            }
          },
          headers: {
            'Proxy-Authorization': 'Basic abc123'