Avoid bad date to be send

[j0k3r]

If the date can't be parsed, it means it's a bad one and shouldn't be return.

For example https://www.helpnetsecurity.com/2015/07/22/its-official-the-average-ddos-attack-size-is-increasing/ has:

<meta property="article:published_time" content="-0001-11-30T00:00:00+00:00" />

[tcitworld]

Is there anything else than ISO 8601 format for dates anyway?

[coveralls]

Coverage decreased (-0.2%) to 97.75% when pulling e9c27ad on fix/bad-date into fad09dd on master.

[j0k3r]

@tcitworld the xpath can return whatever we want. But I think graby shouldn't return a date that can't be parsed as a date