Demonstrate unwanted record sharing.

By adding a pingbacks relationship and not modifying it in any test, we
should end up with an empty array in all serialized Posts, i.e.

    {
        "post": {
            "comments": [
                {
                    "title": "Wouldn't a more lightweight...",
                    "user": null
                },
                {
                    "title": "This does not seem to reflect...",
                    "user": null
                }
            ],
            "pingbacks": [],
            "title": "A New MVC Framework in Under 100 Lines of Code"
        }
    }

However, what we get is

    {
        "post": {
            "comments": [
                {
                    "title": "Wouldn't a more lightweight...",
                    "user": null
                },
                {
                    "title": "This does not seem to reflect...",
                    "user": null
                }
            ],
            "pingbacks": [
                {
                    "title": "Wouldn't a more lightweight...",
                    "user": null
                },
                {
                    "title": "This does not seem to reflect...",
                    "user": null
                }
            ],
            "title": "A New MVC Framework in Under 100 Lines of Code"
        }
    }
Consider relationship for embedded hasMany records.

When serializing a relationship, the previous algorithm serialized all
records in _all_ has-many relationships, not just all records for the
specified relationship.