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/web/api/src/main/resources/definitions/GroupsPost.json b/web/api/src/main/resources/definitions/GroupsPost.json
index f096201..7ea7b50 100644
--- a/web/api/src/main/resources/definitions/GroupsPost.json
+++ b/web/api/src/main/resources/definitions/GroupsPost.json
@@ -19,7 +19,8 @@
     },
     "appCookie": {
       "type": "string",
-      "example": "1"
+      "description": "application cookie. Arbitrary length byte array represented in hex string",
+      "example": "0x1234abcd"
     },
     "groupId": {
       "type": "string",