fix for STANBOL-1339: charset is now appended to the Content-Type header when serializing RDF; STANBOL-1340: application/ld+json is now used as Content-Type for JSON-LD serialied RDF data; also updated integration tests to test both changes

git-svn-id: https://svn.apache.org/repos/asf/stanbol/branches/release-0.12@1594122 13f79535-47bb-0310-9956-ffa450edef68