Undefined values hashing

```javascript
var obj = {
  key: undefined
};
```

When I'm trying to create a hash from object what contains undefined value:

```
TypeError: undefined is not a function
  at Object.dispatch (/Users/harri/Projects/Repos/object-hash/index.js:94:48)
  at /Users/harri/Projects/Repos/object-hash/index.js:114:41
  at Array.forEach (native)
  at _object (/Users/harri/Projects/Repos/object-hash/index.js:111:21)
  at Object.dispatch (/Users/harri/Projects/Repos/object-hash/index.js:94:48)
  at hash (/Users/harri/Projects/Repos/object-hash/index.js:83:31)
  at objectHash (/Users/harri/Projects/Repos/object-hash/index.js:33:10)
  at Test._cb (/Users/harri/Projects/Repos/object-hash/test/index.js:104:15)
  at Test.run (/Users/harri/Projects/Repos/object-hash/node_modules/tape/lib/test.js:56:14)
  at Object.next [as _onImmediate] (/Users/harri/Projects/Repos/object-hash/node_modules/tape/lib/results.js:66:15)
```

Serialising undefined value solves the problem.

Best regards,
Harri