Disable nulls serialization when serialize-nulls property is false

Closes gh-16332

Author: liucheng.zhang

spring-boot-project/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/gson/GsonAutoConfiguration.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2012-2018 the original author or authors.
+ * Copyright 2012-2019 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.
@@ -84,7 +84,8 @@ public void customize(GsonBuilder builder) {
 					.toCall(builder::generateNonExecutableJson);
 			map.from(properties::getExcludeFieldsWithoutExposeAnnotation)
 					.toCall(builder::excludeFieldsWithoutExposeAnnotation);
-			map.from(properties::getSerializeNulls).toCall(builder::serializeNulls);
+			map.from(properties::getSerializeNulls).whenTrue()
+					.toCall(builder::serializeNulls);
 			map.from(properties::getEnableComplexMapKeySerialization)
 					.toCall(builder::enableComplexMapKeySerialization);
 			map.from(properties::getDisableInnerClassSerialization)

spring-boot-project/spring-boot-autoconfigure/src/test/java/org/springframework/boot/autoconfigure/gson/GsonAutoConfigurationTests.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2012-2018 the original author or authors.
+ * Copyright 2012-2019 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.
@@ -81,14 +81,23 @@ public void excludeFieldsWithoutExposeAnnotation() {
 	}
 
 	@Test
-	public void serializeNulls() {
+	public void serializeNullsTrue() {
 		this.contextRunner.withPropertyValues("spring.gson.serialize-nulls:true")
 				.run((context) -> {
 					Gson gson = context.getBean(Gson.class);
 					assertThat(gson.serializeNulls()).isTrue();
 				});
 	}
 
+	@Test
+	public void serializeNullsFalse() {
+		this.contextRunner.withPropertyValues("spring.gson.serialize-nulls:false")
+				.run((context) -> {
+					Gson gson = context.getBean(Gson.class);
+					assertThat(gson.serializeNulls()).isFalse();
+				});
+	}
+
 	@Test
 	public void enableComplexMapKeySerialization() {
 		this.contextRunner