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#### Describe the bug
In v1, accessing the 'Set-Cookie' header in a response interceptor yields the raw string value of the header.

#### To Reproduce

```js
const axios = require('axios'),
      http = require('http');

const server = http
        .createServer((req, res) => {
          res.setHeader('Set-Cookie', 'my=value');
          res.writeHead(200);
          res.write('Hi there');
          res.end();
        })
        .listen(0),
      request = axios.create();

request.interceptors.response.use((res) => {
  console.log(res.headers['set-cookie']);
});

request({ url: `http://localhost:${server.address().port}` }).then(() => server.close());
```

The produces `my=value`

#### Expected behavior
`[ 'my=value' ]`

#### Environment
 - Axios Version [e.g. 0.18.0]: 1.0.0
 - Adapter [e.g. XHR/HTTP]: http
 - Browser [e.g. Chrome, Safari]: N/A
 - Browser Version [e.g. 22]: N/A
 - Node.js Version [e.g. 13.0.1]: 16.13.0
 - OS: [e.g. iOS 12.1.0, OSX 10.13.4]: Ubuntu 20.04
 - Additional Library Versions [e.g. React 16.7, React Native 0.58.0]: N/A

#### Additional context/Screenshots

N/A
lib/core/AxiosHeaders.js
return Object.keys(this).forEach(this.delete.bind(this));
  },
  normalize: function(format) {
    const self = this;
    const headers = {};
    utils.forEach(this, (value, header) => {
      const key = findKey(headers, header);
      if (key) {
        self[key] = normalizeValue(value);
        delete self[header];
        return;
      }
      const normalized = format ? formatHeader(header) : String(header).trim();
      if (normalized !== header) {
        delete self[header];
      }
      self[normalized] = normalizeValue(value);
      headers[normalized] = true;
    });
    return this;
  },
  toJSON: function() {
    const obj = Object.create(null);
    utils.forEach(Object.assign({}, this[$defaults] || null, this),
      (value, header) => {
        if (value == null || value === false) return;
        obj[header] = utils.isArray(value) ? value.join(', ') : value;
      });
    return obj;
  }