GEOS-5844 fix nulls in SecureCatalog.list

a catalog filter can potentially result in nulls being in the
returned iterator. an example is a LocalWorkspaceCatalogFilter
when a WMS virtual service getcaps request occurs.

forward port from 5edc2cc67de9aa922c798c1e8049939178a9d529

had to add means to override check on GeoServerSecurityManager
so test cases would not depend on full spring context
Valid xml
Merge pull request #251 from Gnafu/master

Typo in documentation
[GEOS-5769] JDK 7 gives a preference to the pure java PNG writer even if the native one is available
GEOS-5844 fix nulls in SecureCatalog.list

a catalog filter can potentially result in nulls being in the
returned iterator. an example is a LocalWorkspaceCatalogFilter
when a WMS virtual service getcaps request occurs.
Revert "GEOS-5844 fix nulls in SecureCatalog.list"

This reverts commit b076629ebce1dc4cbdfe2eff622cc361ad0ebc8d.
[GEOS-5854] Add file browser support for GDAL formats
Clean up the kml feature access code and separate it from kmlutils, remove all kmlutils dead methods
GEOS-5849 added localized rule labels support to GetLegendGraphic output
Add support for extended data and schemas in kml
Adding a WFS KML output format
implementing list with vararg sort by, for now only handling single argument
GEOS-5847 fixed issue with nested layergroups
Ensure network links are always getting downloaded, without shrinking the bbox
Merge pull request #253 from mbarto/geos5849_internationalized_getlegendgraphic

GEOS-5849 added localized rule labels support to GetLegendGraphic output
Merge pull request #250 from ischneider/GEOS-5844

GEOS-5844 fix nulls in SecureCatalog.list
GEOS-5851 Make sure we don't synch up computing a non cacheable feature type each time we need a feature source
Fix build
GEOS-5852 default filter chain does not create HTTP session even with session creation enabled
Security documentation enhancments