Improve parsing of dates in user data

[fnesveda]

Currently, we try to parse all response fields ending with At as dates, even in user-supplied data in dataset items, requests in queues etc. This leads to undesired behavior when the data is not parseable as dates.

For example, this request

{
    "url": "http://example.com",
    "userData": {
        "postedAt": "30+ days ago",
    },
}

would get parsed as

{
    url: "http://example.com",
    userData: {
        postedAt: Invalid Date,
    },
}

and there would be no way to get to the actual postedAt field contents.

We could either:

• only parse the date fields if they are actual dates, and keep the original values if they're not (easy, not very breaking)
• not parse dates in user-supplied data at all, parse just data coming from our own data structures (harder, breaking, but feels more proper)