bugfix: Do not erase @context during serialisation when already defined...

bugfix: Do not erase @context during serialisation when already defined (startinblox/djangoldp-packages/djangoldp#153)

when hitting /users/:user-id/,

before:

{
  "@id": "http://localhost:8000/users/chenry/",
   // ...
  "@type": "foaf:user",
  "@context": "https://cdn.happy-dev.fr/owl/hdcontext.jsonld",
  // ...
}

after:

{
  "@id": "http://localhost:8000/users/chenry/",
   // ...
  "@type": "foaf:user",
  "@context": {
    "get_full_name": "rdfs:label"
  },
  // ...
}