fix for when runtime API field name is _ (#1191)

* fix for when runtime API field name is _

* add a test

* formatting

---------

Co-authored-by: Tadeo hepperle <tadeo@do-mix.de>

testing/ui-tests/src/runtime_apis.rs

@@ -0,0 +1,29 @@
+// Copyright 2019-2023 Parity Technologies (UK) Ltd.
+// This file is dual-licensed as Apache-2.0 or GPL-3.0.
+// see LICENSE for license details.
+
+use frame_metadata::{
+    v15::{RuntimeApiMetadata, RuntimeApiMethodMetadata, RuntimeApiMethodParamMetadata},
+    RuntimeMetadataPrefixed,
+};
+
+use crate::utils::generate_metadata_from_runtime_apis;
+
+/// Generate metadata which contains a `Map` storage entry with no hashers/values.
+/// This is a bit of an odd case, but it was raised in https://github.com/paritytech/subxt/issues/552,
+/// and this test will fail before the fix and should pass once the fix is applied.
+pub fn metadata_runtime_api_underscore_method_name() -> RuntimeMetadataPrefixed {
+    generate_metadata_from_runtime_apis(vec![RuntimeApiMetadata {
+        name: "MyApi".to_owned(),
+        docs: vec![],
+        methods: vec![RuntimeApiMethodMetadata {
+            name: "my_method".to_owned(),
+            inputs: vec![RuntimeApiMethodParamMetadata {
+                name: "_".to_owned(), // The important bit we're testing.
+                ty: 0.into(),         // we don't care what type this is.
+            }],
+            output: 0.into(), // we don't care what type this is.
+            docs: vec![],
+        }],
+    }])
+}