Datetime values can have years longer than 4 digits.

[jpbelang]

Continuing issue #600, datetime values have the same issues. This should fail but doesn't.

#%RAML 1.0
title: RAML Proxy
version: v1

types:
    fireworks:
        type: datetime # no implications about offset
        examples:
            bad1: 20161-02-28T16:41:41Z
            bad2: 20161-02-28T16:41:41.090Z

We are headed for a year 10000 problem. You heard it here first :-)