fix: do not iterate over prototype properties of reducers

[danielroe]

reproduction: https://stackblitz.com/edit/stackblitz-starters-iiewc5

import { stringify } from 'devalue';

let a = {};
a.__proto__.appConfigEnabled = 1;

stringify({}, {});
// Error: fn is not a function

[Rich-Harris]

thank you — looks like this causes tests to fail, any ideas?

[danielroe]

apologies for that!

[Rich-Harris]

ah, I see now — thanks!