Micropub: Invalid "published" date silently ignored

[cweiske]

While trying to do this:

Publish a new note via micropub. The note had a published property set to some unix timestamp.

I encountered this error:

The note's published property was silently ignored. The note was created, but with the current timestamp.

Known should reject invalid timestamp values instead of silently ignoring them.

Some other notes:

When setting published to a ISO-861 datetime string like 2016-01-02T23:42:00+02:00, known uses that date correctly.