Undefined values don't get printed

[vthunder]

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
}