AVRO-3631: Add test for serializing fixed fields

This test-case mainly demonstrates the issue reported in AVRO-3631. It
is unclear to me whether we should actually expect the serializer to
serialize to a Value::Fixed right away given that Schema information is
not available at this stage.

lang/rust/avro/src/ser.rs

@@ -676,6 +676,11 @@ mod tests {
         Val2(f32, f32, f32),
     }
 
+    #[derive(Debug, Serialize, Deserialize)]
+    struct TestStructFixedField {
+        field: [u8; 6],
+    }
+
     #[test]
     fn test_to_value() -> TestResult {
         let test = Test {
@@ -1031,4 +1036,18 @@ mod tests {
 
         assert!(ser.is_human_readable());
     }
+
+    #[test]
+    fn test_to_value_fixed_field_avro_3631() {
+        let test = TestStructFixedField { field: [1; 6] };
+        let expected = Value::Record(vec![(
+            "field".to_owned(),
+            Value::Fixed(6, Vec::from(test.field.clone())),
+        )]);
+        assert_eq!(
+            to_value(test).unwrap(),
+            expected,
+            "error serializing fixed array"
+        );
+    }
 }