Add TaggedValue to_value/from_value test

Currently fails with:

    ---- test_tagged stdout ----
    thread 'test_tagged' panicked at 'assertion failed: `(left == right)`
      left: `TaggedValue { tag: !Variant, value: Number(0) }`,
     right: `Mapping {"!Variant": Number(0)}`', tests/test_value.rs:148:5

tests/test_value.rs

@@ -3,7 +3,7 @@
 use indoc::indoc;
 use serde::de::IntoDeserializer;
 use serde::Deserialize;
-use serde_derive::Deserialize;
+use serde_derive::{Deserialize, Serialize};
 use serde_yaml::{Number, Value};
 
 #[test]
@@ -131,3 +131,19 @@ fn test_debug() {
 
     assert_eq!(debug, expected);
 }
+
+#[test]
+fn test_tagged() {
+    #[derive(Serialize)]
+    enum Enum {
+        Variant(usize),
+    }
+
+    let value = serde_yaml::to_value(&Enum::Variant(0)).unwrap();
+
+    let deserialized: serde_yaml::Value = serde_yaml::from_value(value.clone()).unwrap();
+    assert_eq!(value, deserialized);
+
+    let serialized = serde_yaml::to_value(&value).unwrap();
+    assert_eq!(value, serialized);
+}