Allow any ExitCodeGenerator bean to switch of auto-configured bean

In bedf2edf, the return type of the auto-configuration method that
creates batch's ExitCodeGenerator was changed from ExitCodeGenerator
to JobExecutionExitCodeGenerator but the on missing bean condition
was left unchanged. This means that the auto-configured bean can
only be switched off by a JobExecutionExitCodeGenerator bean, rather
than any bean that implements ExitCodeGenerator.

This commit corrects the use of @ConditionalOnMissingBean to allow any
ExitCodeGenerator bean to switch off the auto-configured one.

Closes gh-4752

spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/batch/BatchAutoConfiguration.java

@@ -29,6 +29,7 @@ import org.springframework.batch.core.launch.JobOperator;
 import org.springframework.batch.core.launch.support.SimpleJobOperator;
 import org.springframework.batch.core.repository.JobRepository;
 import org.springframework.beans.factory.annotation.Autowired;
+import org.springframework.boot.ExitCodeGenerator;
 import org.springframework.boot.autoconfigure.AutoConfigureAfter;
 import org.springframework.boot.autoconfigure.EnableAutoConfiguration;
 import org.springframework.boot.autoconfigure.condition.ConditionalOnBean;
@@ -91,7 +92,7 @@ public class BatchAutoConfiguration {
 	}
 
 	@Bean
-	@ConditionalOnMissingBean
+	@ConditionalOnMissingBean(ExitCodeGenerator.class)
 	public JobExecutionExitCodeGenerator jobExecutionExitCodeGenerator() {
 		return new JobExecutionExitCodeGenerator();
 	}