Implemented referencing cached collection.

Change-Id: I02f2ee84b3f8e50c5632efc9363eb87d97a754f8
diff --git a/src/test/resources/collection/unknown-vc-ref.jsonld b/src/test/resources/collection/unknown-vc-ref.jsonld
new file mode 100644
index 0000000..8c7e80e
--- /dev/null
+++ b/src/test/resources/collection/unknown-vc-ref.jsonld
@@ -0,0 +1,4 @@
+{"collection": {
+    "@type": "koral:docGroupRef",
+    "ref": "https://korap.ids-mannheim.de/@ndiewald/MyCorpus"
+}}