RemoteStorage.util.equal not ArrayBuffer-aware

Reproduce:
````js
RemoteStorage.util.equal(new ArrayBuffer('foo', 'UTF-8'), new ArrayBuffer('bar', 'UTF-8'))
RemoteStorage.util.equalObj(new ArrayBuffer('foo', 'UTF-8'), new ArrayBuffer('bar', 'UTF-8'))
````
Expects: `false`, `false`
Observed: `true`, `true`

The reason is that both functions (which we should probably merge into one?) depend on JSON.stringify, which doesn't help to compare ArrayBuffers, because:
````js
JSON.stringify(new ArrayBuffer('bar', 'UTF-8'))
"{}"
````