Resolve needless_borrow clippy lint in test

    error: the borrowed expression implements the required traits
      --> tests/test_serde.rs:22:38
       |
    22 |     let value = serde_yaml::to_value(&thing).unwrap();
       |                                      ^^^^^^ help: change this to: `thing`
       |
       = note: `-D clippy::needless-borrow` implied by `-D clippy::all`
       = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_borrow

tests/test_serde.rs

@@ -19,7 +19,7 @@ where
     let serialized = serde_yaml::to_string(&thing).unwrap();
     assert_eq!(yaml, serialized);
 
-    let value = serde_yaml::to_value(&thing).unwrap();
+    let value = serde_yaml::to_value(thing).unwrap();
     let serialized = serde_yaml::to_string(&value).unwrap();
     assert_eq!(yaml, serialized);