wsu:Created and wsu:Expires should always be in utc format

I've encountered the same issue as in rubiii/savon#29

`Savon::SOAP::Fault: (soap:Client) WSE062: The Timestamp header's <Expires> element is not valid. ---> WSE063: The timestamp 2011-11-03T11:36:10+07:00 in the message is not valid. It must be in this UTC format: yyyy-MM-ddTHH:mm:ssZ.`

This issue was fixed and closed but since then fix was changed and broken. Therefore I've set my mind to fix this issue once again :)

"All times MUST be in UTC format as specified by the XML Schema type
(dateTime)" - excerpt from chapter 10 Security Timestamps http://docs.oasis-open.org/wss/v1.1/wss-v1.1-spec-errata-os-SOAPMessageSecurity.htm