### 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
let proxy = configProxy;
  if (!proxy && proxy !== false) {
    const proxyUrl = getProxyForUrl(location);
    if (proxyUrl) {
      proxy = new URL(proxyUrl);
    }
  }
  if (proxy) {
    
    if (proxy.username) {
      proxy.auth = (proxy.username || '') + ':' + (proxy.password || '');
    }
    if (proxy.auth) {
      
      if (proxy.auth.username || proxy.auth.password) {
        proxy.auth = (proxy.auth.username || '') + ':' + (proxy.auth.password || '');
      }
      const base64 = Buffer
        .from(proxy.auth, 'utf8')
        .toString('base64');
      options.headers['Proxy-Authorization'] = 'Basic ' + base64;
    }
    options.headers.host = options.hostname + (options.port ? ':' + options.port : '');