[GEOS-6627] GWC causes a wrong handling of the ImageIO and ImageIO-Ext plugins

Please look at the following JIRA: https://jira.codehaus.org/browse/GEOS-6627. Note that the following pull request requires that the pull request https://github.com/GeoWebCache/geowebcache/pull/272 is already merged, or that the GeoServer GWC version is upgraded to the 1.7-SNAPSHOT.