serializer for application/nquads should not use long literal

[josephguillaume]

serialize currently outputs quads that use long literals, but the spec apparently does not permit this: Literals may not contain the characters ", LF, or CR. https://www.w3.org/TR/n-quads/#h3_sec-literals

This breaks compatibility with the nquads parser in jsonld.js: https://github.com/digitalbazaar/jsonld.js/issues/44