objects metadata data type: wrong data when no visible field was selected (linked objects)

git-svn-id: http://www.pimcore.org/svn/pimcore/private/core/trunk@2725 1f8fe7d8-47f0-464c-8d0a-336f4953ab05
PIMCORE-1849 - No default creation date saved with objects / unified it for document, asset & object

git-svn-id: http://www.pimcore.org/svn/pimcore/private/core/trunk@2726 1f8fe7d8-47f0-464c-8d0a-336f4953ab05