Make custom converter compatible with latest S-D

The latest changes in the `MappingMongoConverter` resolves the target type from the document.
That type is a `GenericMessage`.
The `GenericConversionService` doesn't match such a target type for the
`MessageReadConverter implements Converter<Document, Message<?>>` and fail.

* Change the `MessageReadConverter` generic type into the `GenericMessage` for matching data in
the collection

Author: artembilan

spring-integration-mongodb/src/test/java/org/springframework/integration/mongodb/store/ConfigurableMongoDbMessageGroupStoreTests.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2019 the original author or authors.
+ * Copyright 2002-2021 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -36,6 +36,7 @@
 import org.springframework.integration.support.MessageBuilder;
 import org.springframework.messaging.Message;
 import org.springframework.messaging.MessageChannel;
+import org.springframework.messaging.support.GenericMessage;
 import org.springframework.util.StopWatch;
 
 /**
@@ -189,14 +190,12 @@ public interface TestGateway {
 	}
 
 	@ReadingConverter
-	public static class MessageReadConverter implements Converter<Document, Message<?>> {
+	public static class MessageReadConverter implements Converter<Document, GenericMessage<?>> {
 
 		@Override
 		@SuppressWarnings("unchecked")
-		public Message<?> convert(Document source) {
-			return MessageBuilder.withPayload(source.get("payload"))
-					.copyHeaders((Map<String, ?>) source.get("headers"))
-					.build();
+		public GenericMessage<?> convert(Document source) {
+			return new GenericMessage<>(source.get("payload"), (Map<String, Object>) source.get("headers"));
 		}
 
 	}