Kotlin JSON 序列化为空？

场景

在尝试使用 Kotlin 结合 SpringBoot 开发后台的过程中，第一部就遇到了序列化的问题。使用https://start.spring.io/ 创建的项目，序列化的库是 Jackson，还需要特别的设置么？

依赖项

plugins {
    id 'org.springframework.boot' version '2.1.3.RELEASE'
    id 'org.jetbrains.kotlin.jvm' version '1.3.21'
    id 'org.jetbrains.kotlin.plugin.spring' version '1.3.21'
}

apply plugin: 'io.spring.dependency-management'

group = 'com.*.*'
version = '0.0.1-SNAPSHOT'
sourceCompatibility = '1.8'

repositories {
    mavenCentral()
}

dependencies {
    implementation 'org.springframework.boot:spring-boot-starter-aop'
    implementation 'org.springframework.boot:spring-boot-starter-web'
    implementation 'com.fasterxml.jackson.module:jackson-module-kotlin'
    implementation 'org.jetbrains.kotlin:kotlin-reflect'
    implementation 'org.jetbrains.kotlin:kotlin-stdlib-jdk8'
    runtimeOnly 'mysql:mysql-connector-java'
    testImplementation 'org.springframework.boot:spring-boot-starter-test'
}

compileKotlin {
    kotlinOptions {
        freeCompilerArgs = ['-Xjsr305=strict']
        jvmTarget = '1.8'
    }
}

compileTestKotlin {
    kotlinOptions {
        freeCompilerArgs = ['-Xjsr305=strict']
        jvmTarget = '1.8'
    }
}

class Essay(private var id: Long?, private var content: String?, private var contentHtml: String?, private var createTime: LocalDateTime?, private var updateTime: LocalDateTime?) {
    override fun toString() = "Essay(id=$id, content=$content, contentHtml=$contentHtml, createTime=$createTime, updateTime=$updateTime)"
}

测试代码

class EssayTest {
    private val log = LoggerFactory.getLogger(this::class.java)
    @Test
    fun test() {
        val essay = Essay(1, "content", "contentHTML", LocalDateTime.now(), LocalDateTime.now())
        log.info("essay: {}", essay)
        val om = ObjectMapper().registerModule(KotlinModule())
        log.info(om.writeValueAsString(essay))
    }
}