next.js/test/integration/basic/pages/custom-encoding.js

import React from 'react'
export default class extends React.Component {
  static async getInitialProps ({res}) {
    if (res) {
      res.setHeader('Content-Type', 'text/html; charset=iso-8859-2')
    }
    return {}
  }

  render () {
    return null
  }
}