### 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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lib/adapters/http.js
} else if (utils.isArrayBuffer(data)) {
        data = Buffer.from(new Uint8Array(data));
      } else if (utils.isString(data)) {
        data = Buffer.from(data, 'utf-8');
      } else {
        return reject(new AxiosError(
          'Data after transformation must be a string, an ArrayBuffer, a Buffer, or a Stream',
          AxiosError.ERR_BAD_REQUEST,
          config
        ));
      }
      
      headers.set('Content-Length', data.length, false);
      if (config.maxBodyLength > -1 && data.length > config.maxBodyLength) {
        return reject(new AxiosError(
          'Request body larger than maxBodyLength limit',
          AxiosError.ERR_BAD_REQUEST,
          config
        ));
      }
    }
    const contentLength = +headers.getContentLength();
    if (utils.isArray(maxRate)) {
      maxUploadRate = maxRate[0];
      maxDownloadRate = maxRate[1];
    } else {
      maxUploadRate = maxDownloadRate = maxRate;
    }
    if (data && (onUploadProgress || maxUploadRate)) {
      if (!utils.isStream(data)) {