Introduce ResolvableType.toClass() shortcut

Issue: SPR-17086

spring-messaging/src/main/java/org/springframework/messaging/handler/annotation/support/MessageMethodArgumentResolver.java

@@ -97,7 +97,7 @@ public class MessageMethodArgumentResolver implements HandlerMethodArgumentResol
 	private Class<?> getPayloadType(MethodParameter parameter) {
 		Type genericParamType = parameter.getGenericParameterType();
 		ResolvableType resolvableType = ResolvableType.forType(genericParamType).as(Message.class);
-		return resolvableType.getGeneric().resolve(Object.class);
+		return resolvableType.getGeneric().toClass();
 	}
 
 	/**

spring-messaging/src/main/java/org/springframework/messaging/handler/invocation/InvocableHandlerMethod.java

@@ -282,7 +282,7 @@ public class InvocableHandlerMethod extends HandlerMethod {
 				return this.returnValue.getClass();
 			}
 			if (!ResolvableType.NONE.equals(this.returnType)) {
-				return this.returnType.resolve(Object.class);
+				return this.returnType.toClass();
 			}
 			return super.getParameterType();
 		}