Rationalize the Reactor autoconfig

It didn't make sense not to do anything at all if a Reactor @Bean
was detected, so now we only back off creating the rootReactor if
one is present.

spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/reactor/ReactorAutoConfiguration.java

@@ -35,11 +35,11 @@ import reactor.spring.context.config.EnableReactor;
  */
 @Configuration
 @ConditionalOnClass(EnableReactor.class)
-@ConditionalOnMissingBean(Reactor.class)
 @AutoConfigureAfter(WebMvcAutoConfiguration.class)
 public class ReactorAutoConfiguration {
 
 	@Bean
+	@ConditionalOnMissingBean(Reactor.class)
 	public Reactor rootReactor(Environment environment) {
 		return environment.getRootReactor();
 	}

spring-boot-autoconfigure/src/test/java/org/springframework/boot/autoconfigure/reactor/ReactorAutoConfigurationTests.java

@@ -16,12 +16,16 @@
 
 package org.springframework.boot.autoconfigure.reactor;
 
+import static org.junit.Assert.assertNotNull;
+
 import org.junit.Test;
 import org.springframework.context.annotation.AnnotationConfigApplicationContext;
+import org.springframework.context.annotation.Bean;
+import org.springframework.context.annotation.Configuration;
 
+import reactor.core.Environment;
 import reactor.core.Reactor;
-
-import static org.junit.Assert.assertNotNull;
+import reactor.core.spec.Reactors;
 
 /**
  * @author Dave Syer
@@ -38,4 +42,21 @@ public class ReactorAutoConfigurationTests {
 		this.context.close();
 	}
 
+	@Test
+	public void customReactor() {
+		this.context.register(TestConfiguration.class, ReactorAutoConfiguration.class);
+		this.context.refresh();
+		assertNotNull(this.context.getBean(Reactor.class));
+		this.context.close();
+	}
+
+	@Configuration
+	protected static class TestConfiguration {
+
+		@Bean
+		public Reactor reactor(Environment env) {
+			return Reactors.reactor().env(env).dispatcher(Environment.RING_BUFFER).get();
+		}
+	}
+
 }