Items with store API key names are considered by key()

[semmel]

Previously the cloaked keys did not get "enumerated" by the key() method:

var storage = new MemoryStorage("my-app");
storage.setItem("getItem", "dummy value");
// returns 1
storage.length

🐛

// returns null
storage.key(0)

✔️

// returns "getItem"
storage.key(0)

[semmel]

semmel@f989601
changes Object.keys(memoryStorage) to conform to Object.keys(localStorage) behaviour in JS environments that support ES6 Proxies.
See #2
Tests validated on Edge 13 and Firefox 44.

[Download]

Your fix is now available in release v0.10.0. Thanks for your help Matthias!