Fix GroupKey interpretation in REST

Input string from REST should not be converted into byte array directly.
Before: "1" -> ascii 49 -> 0x31
After: "0x01" -> 0x01
GroupKey is a byte array with arbitrary length and represented by hex string

Change-Id: If27101f0e5522212c7e434fab58b66e67e9676d7
diff --git a/core/common/src/test/resources/org/onosproject/codec/impl/invalid-group.json b/core/common/src/test/resources/org/onosproject/codec/impl/invalid-group.json
new file mode 100644
index 0000000..c6758cf
--- /dev/null
+++ b/core/common/src/test/resources/org/onosproject/codec/impl/invalid-group.json
@@ -0,0 +1,18 @@
+{
+  "type": "ALL",
+  "deviceId": "of:0000000000000001",
+  "appCookie": "1234abCD",
+  "groupId": "1",
+  "buckets": [
+    {
+      "treatment": {
+        "instructions": [
+          {
+            "type": "OUTPUT",
+            "port": 2
+          }
+        ]
+      }
+    }
+  ]
+}
\ No newline at end of file