feat: allow serialization of unit structs

src/ser/mod.rs

@@ -227,12 +227,12 @@ where
         Err(Error::top_level())
     }
 
-    /// Returns an error.
+    /// Returns `Ok`.
     fn serialize_unit_struct(
         self,
         _name: &'static str,
     ) -> Result<Self::Ok, Error> {
-        Err(Error::top_level())
+        Ok(self.urlencoder)
     }
 
     /// Returns an error.

tests/test_serialize.rs

@@ -73,3 +73,11 @@ fn serialize_unit_enum() {
         Ok("one=A&two=B&three=C".to_owned())
     );
 }
+
+#[derive(Serialize)]
+struct Unit;
+
+#[test]
+fn serialize_unit_struct() {
+    assert_eq!(serde_urlencoded::to_string(Unit), Ok("".to_owned()));
+}