Fix bil bbox

This patch is a fix for issue [GEOS-6662] (http://jira.codehaus.org/browse/GEOS-6662). It fixes a bug in the DDS/BIL plugin which causes WMS 1.3.0 requests to fail with the error "Intersection is null". A unit test is included to ensure that a WMS 1.3.0 request succeeds, and returns the same content as the same request issued using WMS 1.1.1.