fix(rest): sameAs redirect for merged docs (#1537)

sameAs should redirect to the main id for the entity.
This fixes the issue that a deleted (merged) doc with the same id would give 410 DELETED instead

TODO this could/should be implemented as a replacedBy inside the tombstone?

rest/src/main/groovy/whelk/rest/api/Crud.groovy

@@ -338,6 +338,16 @@ class Crud extends HttpServlet {
 
         Document doc = whelk.storage.load(id, version)
         if (doc) {
+            // it has been merged with another doc - main id is in sameAs of remaining doc
+            // TODO this could/should be implemented as a replacedBy inside the tombstone?
+            if (doc.deleted && !JsonLd.looksLikeIri(id)) {
+                String iri = Document.BASE_URI.toString() + id + Document.HASH_IT
+                String location = whelk.storage.getMainId(iri)
+                if (location) {
+                    return new Tuple2(null, location)
+                }
+            }
+
             return new Tuple2(doc, null)
         }