INT-1926: Option to disallow arbitrary routing

JIRA: https://jira.spring.io/browse/INT-1926

Add an option to mapping routers to disable falling back to the
channel key as the channel name.

spring-integration-core/src/test/java/org/springframework/integration/dsl/routers/NoFallbackAllowedTests.java

@@ -0,0 +1,68 @@
+/*
+ * Copyright 2019 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.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.integration.dsl.routers;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+import java.util.Collections;
+
+import org.junit.jupiter.api.Test;
+
+import org.springframework.beans.factory.annotation.Autowired;
+import org.springframework.context.annotation.Bean;
+import org.springframework.context.annotation.Configuration;
+import org.springframework.integration.channel.QueueChannel;
+import org.springframework.integration.config.EnableIntegration;
+import org.springframework.integration.dsl.IntegrationFlow;
+import org.springframework.messaging.support.GenericMessage;
+import org.springframework.test.context.junit.jupiter.SpringJUnitConfig;
+
+/**
+ * @author Gary Russell
+ * @since 5.2
+ *
+ */
+@SpringJUnitConfig
+public class NoFallbackAllowedTests {
+
+	@Test
+	void noStackOverflow(@Autowired Config config) {
+		config.flow()
+				.getInputChannel()
+				.send(new GenericMessage<>("foo", Collections.singletonMap("whereTo", "flow.input")));
+		assertThat(config.queue().receive(0)).isNotNull();
+	}
+
+	@Configuration
+	@EnableIntegration
+	public static class Config {
+
+		@Bean
+		public IntegrationFlow flow() {
+			return f -> f.route("headers.whereTo", r -> r
+					.noChannelKeyFallback()
+					.defaultOutputChannel(queue()));
+		}
+
+		@Bean
+		public QueueChannel queue() {
+			return new QueueChannel();
+		}
+
+	}
+
+}