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#### 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**

```js
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.



lib/core/dispatchRequest.js
/**
 * Dispatch a request to the server using the configured adapter.
 *
 * @param {object} config The config that is to be used for the request
 * @returns {Promise} The Promise to be fulfilled
 */
module.exports = function dispatchRequest(config) {
  throwIfCancellationRequested(config);
  
  config.headers = config.headers || {};
  
  config.data = transformData.call(
    config,
    config.data,
    config.headers,
    null,
    config.transformRequest
  );
  
  config.headers = utils.merge(
    config.headers.common || {},
    config.headers[config.method] || {},
    config.headers
  );
  utils.forEach(
    ['delete', 'get', 'head', 'post', 'put', 'patch', 'common'],
    function cleanHeaderConfig(method) {
      delete config.headers[method];
    }
  );