Describe the bug

when 'content-type' is in lowercase，
Devtool Network show :Content-Type: application/x-www-form-urlencoded, application/x-www-form-urlencoded，
that is repeated.

To Reproduce

Code snippet to reproduce, ideally that will work by pasting into something like https://npm.runkit.com/axios, a hosted solution, or a repository that illustrates the issue. If your problem is not reproducible, please file under Support or Usage Question

axios({
  method: 'post',
  url: 'http://yapi.corp.lanyicj.cn/mock/228/stock_trade/user_account',
  headers: { 'content-type': 'application/x-www-form-urlencoded' },
  transformRequest: [
    function () {
      return 'aa=44'
    }
  ]
})

Expected behavior

Content-Type: application/x-www-form-urlencoded

Environment

• Axios Version [e.g. 0.18.0]
• Adapter [e.g. XHR/HTTP]
• Browser [e.g. Chrome, Safari]
• Browser Version [e.g. 22]
• Node.js Version [e.g. 13.0.1]
• OS: [e.g. iOS 12.1.0, OSX 10.13.4]
• Additional Library Versions [e.g. React 16.7, React Native 0.58.0]

Additional context/Screenshots

Add any other context about the problem here. If applicable, add screenshots to help explain.