Fix date parser in ISO8601

[lesiak]

Problematic example
2018-11-11 13:23:52 UTC

I receive this format in webhooks.
See gitlab issue https://gitlab.com/gitlab-org/gitlab-ce/issues/38138
(Inconsistent date time formats across the API and webhooks)

Broken in: 4.10.0

[gmessner]

@lesiak
I've added proper handling of UTC zoned dates to ISO8601 and have released 4.10.3 with the fix and additional tests. Please verify and close this issue when you have time.

[lesiak]

4.10.3 works like charm - it passes all our test cases.
Thanks for solving the issue so quickly!