STANBOL-1236: JSON-LD serializer now uses 'application/ld+json' as mediatype. 'application/json' is still supported (for backward compatibility) but we might want to remove this at a later point

git-svn-id: https://svn.apache.org/repos/asf/stanbol/trunk@1551140 13f79535-47bb-0310-9956-ffa450edef68