1123 made CompositeMessageChannelConfigurer conditional

Annotated CompositeMessageChannelConfigurer wih @ConditionalOnMissingBean to allow it to be overriden

Resolves #1123

spring-cloud-stream/src/main/java/org/springframework/cloud/stream/config/BindingServiceConfiguration.java

@@ -140,6 +140,7 @@ public class BindingServiceConfiguration {
 	}
 
 	@Bean
+	@ConditionalOnMissingBean
 	public CompositeMessageChannelConfigurer compositeMessageChannelConfigurer(
 			MessageConverterConfigurer messageConverterConfigurer) {
 		List<MessageChannelConfigurer> configurerList = new ArrayList<>();