use UTC when @timestamp is explicetely specified

Previously logstash used utc timestamps. After commit 1149aa34c779f54c429e90a724d828648b9d61a2 (use ISODateTimeFormat for parsing @timestamp strings), explicitly specified timestamps started to be displayed in the local timezone (2013-12-12T19:58:36.193+03:00), although the message contains timestamp in the utc timezone (2013-12-12T19:58:36.193Z)

The following patch fixes this issue.