Add toRdf test for expand e051.

Author: davidlehn

tests/toRdf-manifest.jsonld

@@ -1099,6 +1099,13 @@
       "purpose": "Verifies that an exception is raised in Expansion when an invalid IRI is used for @reverse.",
       "input": "toRdf/ee50-in.jsonld",
       "expectErrorCode": "invalid IRI mapping"
+    }, {
+      "@id": "#tee51",
+      "@type": ["jld:NegativeEvaluationTest", "jld:ToRDFTest"],
+      "name": "Invalid value object value using a value alias",
+      "purpose": "Verifies that an exception is raised in Expansion when an invalid value object value is found using a value alias",
+      "input": "toRdf/ee51-in.jsonld",
+      "expectErrorCode": "invalid value object value"
     }, {
       "@id": "#te051",
       "@type": ["jld:PositiveEvaluationTest", "jld:ToRDFTest"],

tests/toRdf/ee51-in.jsonld

@@ -0,0 +1,4 @@
+{
+  "@context": {"value": "@value"},
+  "http://example/prop": {"value": ["foo"]}
+}