Issue #242 Exception thrown when calling getCoverageIntersections on a GridSubset. (Backport to 1.5.x branch)

Backport of the pill request https://github.com/GeoWebCache/geowebcache/pull/243 on the 1.5.x branch.

Fixed bug on the GridSubSet class. The related issue is https://github.com/GeoWebCache/geowebcache/issues/242.

A test class is associated.