Undefined values don't get printed

When pretty-printing an object, keys with undefined values don't get printed.

e.g.:

```
let obj = {foo: "bar", baz: undefined};
chai.expect(obj, obj).to.deep.equal({foo: "bar"});
```

will pretty-print:

```
{
 "foo": "bar"
}
```

it should print instead:

```
{
  "foo": "bar",
  "baz": undefined
}
```
