Ensure ApplicationContextInitializers are not overwritten

SpringApplicationBuilder has to be careful not to overwrite the
default initializers if user calls its initializers() method.

Fixes gh-120.

spring-boot/src/main/java/org/springframework/boot/builder/SpringApplicationBuilder.java

@@ -481,7 +481,8 @@ public class SpringApplicationBuilder {
 			target.addAll(Arrays.asList(initializers));
 		}
 		this.initializers = target;
-		this.application.setInitializers(target);
+		this.application.addInitializers(target
+				.toArray(new ApplicationContextInitializer[0]));
 	}
 
 }

spring-boot/src/test/java/org/springframework/boot/builder/SpringApplicationBuilderTests.java

@@ -23,6 +23,7 @@ import java.util.Collections;
 import org.junit.After;
 import org.junit.Test;
 import org.springframework.context.ApplicationContext;
+import org.springframework.context.ApplicationContextInitializer;
 import org.springframework.context.ConfigurableApplicationContext;
 import org.springframework.context.annotation.AnnotationConfigApplicationContext;
 import org.springframework.context.annotation.Configuration;
@@ -178,6 +179,20 @@ public class SpringApplicationBuilderTests {
 		assertEquals(7, application.application().getInitializers().size());
 	}
 
+	@Test
+	public void initializersIncludeDefaults() throws Exception {
+		SpringApplicationBuilder application = new SpringApplicationBuilder(
+				ExampleConfig.class).web(false).initializers(
+				new ApplicationContextInitializer<ConfigurableApplicationContext>() {
+					@Override
+					public void initialize(
+							ConfigurableApplicationContext applicationContext) {
+					}
+				});
+		this.context = application.run();
+		assertEquals(8, application.application().getInitializers().size());
+	}
+
 	@Configuration
 	static class ExampleConfig {