JSON-LD: improve context handling on framing and compacting

[GGoetzelmann]

Before PR:

• contexts of frame are not provided to compacting operation and therefore not explicitely provided in compacted output, potentially breaking the json-ld payload representation (the problem does not occur as long as the original frame file is not touched)

After PR:

• contexts for jsonld payload can now come from
  • frame
  • profiles
• contexts will be deduplicated, then used for compacting
• minor (breaking) effects on content negotiation: since compacting is now dependent on all contexts, not only on profiles, compacting is applied in more cases than before

This PR aims to allow customization of frames, for example to permanently include additional context information in all responses of a wap server instance, effectively allowing for more customized context handling without the risk of breaking the anno.jsonld profile in the process