[bugfix] handle null prototype objects

[magicmark]

@mcmath thanks for this library!

Came across an issue we're hitting - if this library called over any objects created with Object.create(null), then an error is thrown because of this line:

deep-map-keys/src/deep-map-keys.ts

Line 58 in 4a22045

if (obj.hasOwnProperty(key)) {

I've added a regression test, which also demonstrates the issue:

before

$ npm run test:unit

> deep-map-keys@2.0.1 test:unit
> istanbul cover -e .ts -x '*.test.ts' _mocha -- src --opts mocha.opts

  !․․․․․․․․․․․․․

  13 passing (39ms)
  1 failing

  1) handles null prototype objects:
     TypeError: obj.hasOwnProperty is not a function
      at DeepMapKeys.mapObject (src/deep-map-keys.ts:1:6130)
      at DeepMapKeys.map (src/deep-map-keys.ts:1:5022)
      at deepMapKeys (src/index.ts:1:3000)
      at Context.<anonymous> (src/index.test.ts:112:3)
      at processImmediate (node:internal/timers:471:21)

after

it passes!