Implement __typename for GraphQL objects

Fixes #22.

src/macros/object.rs

@@ -378,6 +378,10 @@ macro_rules! graphql_object {
                 mt.into_meta()
             }
 
+            fn concrete_type_name(&self, _: &Self::Context) -> String {
+                $outname.to_owned()
+            }
+
             #[allow(unused_variables)]
             #[allow(unused_mut)]
             fn resolve_field(

src/tests/query_tests.rs

@@ -362,3 +362,24 @@ fn test_query_inline_fragments_human() {
             ].into_iter().collect()),
             vec![])));
 }
+
+#[test]
+fn test_object_typename() {
+    let doc = r#"
+        {
+            human(id: "1000") {
+                __typename
+            }
+        }"#;
+    let database = Database::new();
+    let schema = RootNode::new(&database, EmptyMutation::<Database>::new());
+
+    assert_eq!(
+        ::execute(doc, None, &schema, &Variables::new(), &database),
+        Ok((Value::object(vec![
+                ("human", Value::object(vec![
+                    ("__typename", Value::string("Human")),
+                ].into_iter().collect())),
+            ].into_iter().collect()),
+            vec![])));
+}