Don't apply `null` ServerProperties from customize

Update ServerProperties so that `null` values are not applied when
customizing the EmbeddedServletContainerFactory. Primarily changed to
stop `server.undertow.accesslog.enabled` from being blindly applied.

Fixes gh-5515

spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/web/ServerProperties.java

 /*
- * Copyright 2012-2015 the original author or authors.
+ * Copyright 2012-2016 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.
@@ -1019,7 +1019,7 @@ public class ServerProperties
 		@Deprecated
 		@DeprecatedConfigurationProperty(replacement = "server.undertow.accesslog.enabled")
 		public boolean isAccessLogEnabled() {
-			return this.accesslog.isEnabled();
+			return Boolean.TRUE.equals(this.accesslog.getEnabled());
 		}
 
 		/**
@@ -1055,14 +1055,30 @@ public class ServerProperties
 
 		void customizeUndertow(ServerProperties serverProperties,
 				UndertowEmbeddedServletContainerFactory factory) {
-			factory.setBufferSize(this.bufferSize);
-			factory.setBuffersPerRegion(this.buffersPerRegion);
-			factory.setIoThreads(this.ioThreads);
-			factory.setWorkerThreads(this.workerThreads);
-			factory.setDirectBuffers(this.directBuffers);
-			factory.setAccessLogDirectory(this.accesslog.dir);
-			factory.setAccessLogPattern(this.accesslog.pattern);
-			factory.setAccessLogEnabled(this.accesslog.enabled);
+			if (this.bufferSize != null) {
+				factory.setBufferSize(this.bufferSize);
+			}
+			if (this.buffersPerRegion != null) {
+				factory.setBuffersPerRegion(this.buffersPerRegion);
+			}
+			if (this.ioThreads != null) {
+				factory.setIoThreads(this.ioThreads);
+			}
+			if (this.workerThreads != null) {
+				factory.setWorkerThreads(this.workerThreads);
+			}
+			if (this.directBuffers != null) {
+				factory.setDirectBuffers(this.directBuffers);
+			}
+			if (this.accesslog.dir != null) {
+				factory.setAccessLogDirectory(this.accesslog.dir);
+			}
+			if (this.accesslog.pattern != null) {
+				factory.setAccessLogPattern(this.accesslog.pattern);
+			}
+			if (this.accesslog.enabled != null) {
+				factory.setAccessLogEnabled(this.accesslog.enabled);
+			}
 			factory.setUseForwardHeaders(serverProperties.getOrDeduceUseForwardHeaders());
 		}
 
@@ -1071,7 +1087,7 @@ public class ServerProperties
 			/**
 			 * Enable access log.
 			 */
-			private boolean enabled = false;
+			private Boolean enabled;
 
 			/**
 			 * Format pattern for access logs.
@@ -1083,11 +1099,11 @@ public class ServerProperties
 			 */
 			private File dir = new File("logs");
 
-			public boolean isEnabled() {
+			public Boolean getEnabled() {
 				return this.enabled;
 			}
 
-			public void setEnabled(boolean enabled) {
+			public void setEnabled(Boolean enabled) {
 				this.enabled = enabled;
 			}
 

spring-boot-autoconfigure/src/test/java/org/springframework/boot/autoconfigure/web/ServerPropertiesTests.java

 /*
- * Copyright 2012-2015 the original author or authors.
+ * Copyright 2012-2016 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.
@@ -53,6 +53,7 @@ import static org.junit.Assert.assertFalse;
 import static org.junit.Assert.assertNull;
 import static org.junit.Assert.assertThat;
 import static org.mockito.BDDMockito.given;
+import static org.mockito.Matchers.anyBoolean;
 import static org.mockito.Mockito.atLeastOnce;
 import static org.mockito.Mockito.mock;
 import static org.mockito.Mockito.never;
@@ -412,6 +413,14 @@ public class ServerPropertiesTests {
 		verify(container).setSessionStoreDir(new File("myfolder"));
 	}
 
+	@Test
+	public void skipNullElementsForUndertow() throws Exception {
+		UndertowEmbeddedServletContainerFactory container = mock(
+				UndertowEmbeddedServletContainerFactory.class);
+		this.properties.customize(container);
+		verify(container, never()).setAccessLogEnabled(anyBoolean());
+	}
+
 	private void bindProperties(Map<String, String> map) {
 		new RelaxedDataBinder(this.properties, "server")
 				.bind(new MutablePropertyValues(map));