Fix sp-api handling of multiple arguments (#6484)

With the switch to `decode_all_with_depth_limit` we silently broken
support for functions with multiple arguments. The old generated code
tried to decode each parameter separately, which does not play well with
`decode_all`.

This pr adds a test to ensure that this does not happen again and fixes
the bug by decoding everything at once by wrapping it into tuples.

substrate/primitives/api/test/tests/runtime_calls.rs

@@ -207,3 +207,14 @@ fn record_proof_works() {
 		&runtime_code,
 	).expect("Executes block while using the proof backend");
 }
+
+#[test]
+fn call_runtime_api_with_multiple_arguments() {
+	let client = TestClientBuilder::new().set_execution_strategy(ExecutionStrategy::Both).build();
+
+	let data = vec![1, 2, 4, 5, 6, 7, 8, 8, 10, 12];
+	let block_id = BlockId::Number(client.chain_info().best_number);
+	client.runtime_api()
+		.test_multiple_arguments(&block_id, data.clone(), data.clone(), data.len() as u32)
+		.unwrap();
+}