fix: Unable to access response headers when using axios 1.x with jest #5017

[risa]

When axios 1.0.0 runs under jest, it fails to set headers on the response object properly.

For a detailed description please see here: #5017

The problem in the plainObject check function, where

prototype === null || prototype === Object.prototype;

Is false for response.headers under jest, as jest seems to modify prototype of objects.

The fix is inspired by the is plain module https://github.com/sindresorhus/is-plain-obj, mainly their checking logic:

https://github.com/sindresorhus/is-plain-obj/blob/68e8cc77bb1bbd0bf7d629d3574b6ca70289b2cc/index.js#L7

Edit: I do now know how to write a test for this, as it manifests only under a different test framework than the project uses.