Handle iso format where Z is prefixed with spaces

[ichernev]

In response to #1290

[Xotic750]

As far as I am aware, a space before a 'Z' is not a valid iso 8601 format. While it may be worth being able to pass such an invalid stamp. shouldn't moment mark this as inValid aswell?

UTC[edit]
If the time is in UTC, add a Z directly after the time without a space. Z is the zone designator for the zero UTC offset. "09:30 UTC" is therefore represented as "09:30Z" or "0930Z". "14:45:15 UTC" would be "14:45:15Z" or "144515Z".
UTC time is also known as 'Zulu' time, since 'Zulu' is the NATO phonetic alphabet word for 'Z'.

[icambron]

I think it probably isn't valid ISO-8601, but it used to work and it's advertised to work. If we remove it, we should remove it in 3.0.

Re: invalid: we've defined val to mean "moment understands what the date is", so if we can parse it into an actual real date, it's valid.