Avoid extracting resoure set from backing range set

This is one of fixes for ONOS-4684

Change-Id: I5192df366d16b8aad5f8987abbf539897ca0e51c
diff --git a/core/store/dist/src/test/java/org/onosproject/store/resource/impl/EncodedDiscreteResourcesTest.java b/core/store/dist/src/test/java/org/onosproject/store/resource/impl/EncodedDiscreteResourcesTest.java
index fe1e781..066f57f 100644
--- a/core/store/dist/src/test/java/org/onosproject/store/resource/impl/EncodedDiscreteResourcesTest.java
+++ b/core/store/dist/src/test/java/org/onosproject/store/resource/impl/EncodedDiscreteResourcesTest.java
@@ -62,4 +62,17 @@
         assertThat(sut.difference(other), is(EncodedDiscreteResources.of(ImmutableSet.of(res2), new VlanIdCodec())));
     }
 
+    @Test
+    public void testAdd() {
+        DiscreteResource res1 = Resources.discrete(DID, PN, VID1).resource();
+        DiscreteResource res2 = Resources.discrete(DID, PN, VID2).resource();
+        DiscreteResource res3 = Resources.discrete(DID, PN, VID3).resource();
+
+        EncodedDiscreteResources sut = EncodedDiscreteResources.of(ImmutableSet.of(res1, res2), new VlanIdCodec());
+        EncodedDiscreteResources other = EncodedDiscreteResources.of(ImmutableSet.of(res1, res3), new VlanIdCodec());
+
+        assertThat(sut.add(other),
+                is(EncodedDiscreteResources.of(ImmutableSet.of(res1, res2, res3), new VlanIdCodec())));
+    }
+
 }