Different final header between two versions

[WhoisUnknown]

Describe the bug
I have different result header request in 2 versions 18.1 and 19.2
This is not in changelog

To Reproduce
Create axios object and made any request

import ax from 'axios';
const axios = ax.create({
  headers: {
    LOCALE: ['ru'],
  },
});

Expected behavior
Same result header

Environment:

• Axios Version [e.g. 0.18.0]
• OS: Linux Mint
• Browser: Chrome
• Browser Version 80.0.3987.132
• Additional Library Versions Vue 2.6.11

In 18.1

In 19.2