item entity doesn't contain always context property

[guenterh]

It seems that not all context items (on the level of ElasticSearch) contain the context property.

If the property isn't available it sho uldn't be possible to parse the subjects in question with RDF parsers

• make tests
• why isn't the context always generated on the level of MF?

{

"_index": "lsb_20170718",
"_type": "item",
"_id": "IDSBB-76d120ae-3b85-32d6-a518-1902b6a68f90",
"_score": 1.0,
"_source": {
    "bf:subLocation": "Volltext nur im Uninetz zugänglich. Link siehe unter 'Online'",
    "@id": "http://data.swissbib.ch/item/IDSBB-76d120ae-3b85-32d6-a518-1902b6a68f90",
    "bf:holdingFor": "http://data.swissbib.ch/bibliographicResource/112182992",
    "foaf:page": "http://baselbern.swissbib.ch/Record/112182992?expandlib=B405#holding-institution-IDSBB-B405",
    "bibo:owner": "http://data.swissbib.ch/organisation/IDSBB-B405"
}

},
{

"_index": "lsb_20170718",
"_type": "item",
"_id": "IDSBB-41b9ca7e-91e5-302b-9df7-4ecc9e16aa73",
"_score": 1.0,
"_source": {
    "@type": "http://bibframe.org/vocab/HeldItem",
    "@context": "https://resources.swissbib.ch/item/context.jsonld",
    "bf:subLocation": "Volltext nur im Uninetz zugänglich. Link siehe unter 'Online'",
    "@id": "http://data.swissbib.ch/item/IDSBB-41b9ca7e-91e5-302b-9df7-4ecc9e16aa73",
    "bf:holdingFor": "http://data.swissbib.ch/bibliographicResource/112183034",
    "foaf:page": "http://baselbern.swissbib.ch/Record/112183034?expandlib=A145#holding-institution-IDSBB-A145",
    "bibo:owner": "http://data.swissbib.ch/organisation/IDSBB-A145"
}

},