This is *usually* what you want (and is defaulted this way). If you have an entry in an array *just* so it can become the key to that entry later, then you shouldn't normally still need it in the resulting array.
The importance of this comes in with validation. Since we're throwing an exception if you have any unrecognized options, the presence of the "key" field in the resulting array will cause issues when it's not needed.
