GEOS-5710 GetLegendGraphic for a layergroup with a coverage invisble at current scale fails
cache-control headers in integrated wms, GEOS-5686

- cleanup interceptor, add max-age header from layer metadata if
  declared
- add test

Conflicts:

	src/gwc/src/test/java/org/geoserver/gwc/GWCIntegrationTest.java
Merge pull request #189 from mbarto/geos_5710

GEOS-5710 GetLegendGraphic for a layergroup with a coverage invisble at current scale fails
Merge pull request #190 from ischneider/GEOS-5686

cache-control headers in integrated wms, GEOS-5686