GEOS-6258 - Not considering http error codes < 400 as an error.

Patch provided by Rob Marianski.
make application context for test framework more consistent with runtime

backport of #441.
Backporting the pngj module to the 2.3.x series
Merge branch '2.3.x' of https://github.com/geoserver/geoserver into 2.3.x