test, and clean up, support for __typename

generate/testdata/TypeName.graphql

@@ -0,0 +1,6 @@
+{
+  user {
+    __typename
+    id
+  }
+}

generate/testdata/TypeName.graphql.go

@@ -0,0 +1,6 @@
+type Response struct {
+	User *struct {
+		Typename *string `json:"__typename"`
+		Id       string  `json:"id"`
+	} `json:"user"`
+}