Default methods Content-Type headers cant be overridden

Describe the bug When setting default Content-Type for methods (e.g. patch) always sends ContentType: application/json;charset=utf-8

To Reproduce

const api = axios.create({
  baseURL: '/api',
  headers: {
    common: {
      Accept: 'application/json'
    },
    patch: {
      'Content-Type': 'application/merge-patch+json'
    }
  }
})

api.patch('/user/1', { foo: 'bar' })

Expected behavior Should send Content-Type: application/merge-patch+json header.

Environment:

• Axios Version 0.19.0
• OS: macOS 10.15.1
• Browser Chrome, Safari
• Browser Version 78.0.3904.108

Additional context/Screenshots I think problem is here – root headers from config.headers always overrides headers for specific methods. In example above config.headers['Content-Type'] not set, so axios set it to application/json;charset=utf-8 under the hood and config.headers['Content-Type'] overrides config.headers.patch['Content-Type']. I think methods headers should have higher priority and should be passed to utils.merge function in last argument

[pelepelin]

@WalderlanSena your example uses explicit headers settings for the request. This issue is about defaults provided to axios.create.

[pelepelin]

Workaround

Apply a custom transformRequest suitable for your usecase instead.

require('axios').create({
  transformRequest: (data, headers) => {
    if (data !== undefined) {
      headers['Content-Type'] = 'application/json';

      return JSON.stringify(data)
    } else {
      return data;
    }
  }
}).request({
  url: 'http://httpbin.org/anything',
  method: 'post',
  data: {}
}).then(response => console.log(response.data.headers['Content-Type']));

Result:

application/json