WMS time extent does not properly encode dates with negative years

• Testcase included:

  yes
• Patch Submitted:

  Yes

java.util.Date supports representation of dates between -296,000,000 to 296,000,000 years.

The current implementation of date output in the getcapabilities response for time extent does not properly encode these values according to the WMS specification (section D2.2, pp 57-58).

Attached patch ensures this with a new formatter (SimpleDateFormat cannot support this format directly) and verifies compliance with the extended ISO8601 format in the TimeKvpParserTest.