Add packed serialized test for struct

See issue #144

tests/value.rs

@@ -80,4 +80,21 @@ mod std_tests {
 
         assert_eq!(value, data_de_value);
     }
+
+    #[derive(Debug, Deserialize, Serialize)]
+    struct SmallStruct {
+        spam: u32,
+        eggs: u32,
+    }
+
+    #[test]
+    fn small_struct() {
+        // Test whether the packed format works.
+        // Field names should not be serialized,
+        // instead field indizes are serialized.
+        let value = SmallStruct { spam: 17, eggs: 42 };
+        let data = serde_cbor::ser::to_vec_packed(&value).unwrap();
+        let reference = b"\xa2\x00\x11\x01\x18\x2a";
+        assert_eq!(data, reference);
+    }
 }