fix object to null comparison

Comparing an object with at least one key against `null` resulted in

```
TypeError: Cannot call method 'hasOwnProperty' of null
    at eql (/code/lib/mongo-eql/index.js:50:20)
    at Object.<anonymous> (/code/lib/mongo-eql/test.js:4:8)
    at Module._compile (module.js:456:26)
    at Object.Module._extensions..js (module.js:474:10)
    at Module.load (module.js:356:32)
    at Function.Module._load (module.js:312:12)
    at Function.Module.runMain (module.js:497:10)
    at startup (node.js:119:16)
    at node.js:906:3
```

Test included.