Write trailing newline after yaml document

Unlike with JSON, there is never really a use case for the caller to
write more stuff on the same line after the last character of YAML
output.

src/lib.rs

@@ -29,7 +29,7 @@
 //!
 //!     // Serialize it to a YAML string.
 //!     let s = serde_yaml::to_string(&map)?;
-//!     assert_eq!(s, "---\nx: 1.0\ny: 2.0");
+//!     assert_eq!(s, "---\nx: 1.0\ny: 2.0\n");
 //!
 //!     // Deserialize it back to a Rust type.
 //!     let deserialized_map: BTreeMap<String, f64> = serde_yaml::from_str(&s)?;
@@ -57,7 +57,7 @@
 //!     let point = Point { x: 1.0, y: 2.0 };
 //!
 //!     let s = serde_yaml::to_string(&point)?;
-//!     assert_eq!(s, "---\nx: 1.0\ny: 2.0");
+//!     assert_eq!(s, "---\nx: 1.0\ny: 2.0\n");
 //!
 //!     let deserialized_point: Point = serde_yaml::from_str(&s)?;
 //!     assert_eq!(point, deserialized_point);

src/ser.rs

@@ -396,6 +396,7 @@ where
     YamlEmitter::new(&mut writer_adapter)
         .dump(&doc)
         .map_err(error::emitter)?;
+    writer_adapter.writer.write_all(b"\n").map_err(error::io)?;
     Ok(())
 }
 

tests/test_serde.rs

@@ -16,7 +16,6 @@ fn test_serde<T>(thing: &T, yaml: &str)
 where
     T: serde::Serialize + serde::de::DeserializeOwned + PartialEq + Debug,
 {
-    let yaml = yaml.trim_end_matches('\n');
     let serialized = serde_yaml::to_string(&thing).unwrap();
     assert_eq!(yaml, serialized);