Missing properties not equal to undefined

Is this intended?
```js
dequal({ x: undefined}, {}); //=> false
// Returns false, even though arg1.x === arg2.x
```

If it is, could it be documented?

My use case is to check for unsaved changes by comparing a current value with an original value. But to save that original value, I clone it with `JSON.stringify`, which doesn't work because of this issue

Start by reproducing the dequal({ x: undefined }, {}) example and reviewing how the comparison treats missing properties. Decide whether this behavior is intended; if so, document it, or otherwise define the expected equality behavior and add coverage for the reported case. JSON.stringify cloning is part of the reported use case.

Written by the indexing model from the issue text.