{ $exists: false ] FAILS

console.log(query.filter({ a: 3 }, { a: { $exists: true } } )); // yields {}
console.log(query.filter({ a: 3 }, { b: { $exists: false } } )); // yields false

filter.js line 94, is the root cause:

```
case 'undefined':
// if we can't find the key
return false;
```

Start at filter.js line 94 and reproduce the two console examples to trace how a missing key is handled. Check the $exists true and false cases against the reported results; done means both examples return the expected match or non-match instead of {} or false.

Written by the indexing model from the issue text.