Add a test to show a bug on the date-time format constraint checker

[mathroc]

it was supposed to be solved in #207 like that:

// handles the case where a non-6 digit microsecond datetime is passed
// which will fail the above string comparison because the passed
// $datetime may be '2000-05-01T12:12:12.123Z' but format() will return
// '2000-05-01T12:12:12.123000Z'
if ((strpos('u', $format) !== -1) && (intval($dt->format('u')) > 0)) {
    return true;
}

(see https://github.com/justinrainbow/json-schema/pull/207/files#diff-94f691bae259df522f96670badc55b60R141)

but I'm not sure this is valid and I don't know how to solve it correctly easily. So for now this PR just shows the bug, I'll try to solve it later. if anyone have a suggestion, I'm all ears

maybe like mentioned on #145 ?

[vaeryn-uk]

Ah, sorry. I hadn't realised that a PR was open for this already. I've created a fix if it's of any use: #234.

Not overly happy with the solution, but couldn't come up with anything better.

[bighappyface]

Closed with #238

Please re-open if necessary.