Summary

Headers provided in config are merged with defaults without checking for existing value that can be in different case

i.e. when doing below request we can't predict which value will be set from defaults Accept or from config accept

axios.request(url, {
  headers: {
    accept: 'some-accept-value'
  }
});

Chrome will set Accept, which is not expected

Context

• axios version: 0.17.0
• Environment: node v9.2.1, chrome 63, macOS