Diff of objects missing function parameters

Might be somewhat related to #51

> input
```js
assert.equal(
    { foo: 'bar', baz: () => {} },
    { foo: 'bar' }
);
```

> output
```
Expected values to be deeply equal:  (equal)

    ··{
    ····"foo":·"bar"
    ··}
```

Some print out would be nice, even if it's rather quick to debug and figure out what's going on.