SpringMVC+Jackson为什么抛出406错误?

Spring:4.3.8
​Jackson:2.8.8

HTML报的错误：The resource identified by this request is only capable of generating responses with characteristics not acceptable according to the request "accept" headers.  

异常抛出的位置：

AbstractMessageConverterMethodProcessor类中的

if (outputValue != null && producibleMediaTypes.isEmpty()) {
   throw new IllegalArgumentException("No converter found for return value of type: " + valueType);
} 其中:outputValue:LoginReply@6431 producibleMediaTypes: size=4

SpringMvc配置

<bean class="org.springframework.web.servlet.view.InternalResourceViewResolver" p:suffix=".jsp"
      p:prefix="/WEB-INF/jsp/"/>
<context:component-scan base-package="com.fresh.web"/>
<!-- Enables the Spring MVC @Controller programming model -->
<!--<bean id="conversionService" class="org.springframework.format.support.FormattingConversionServiceFactoryBean">-->
    <!--<property name="converters">-->
        <!--<list>-->
            <!--<bean class="com.fresh.conversion.CustomDateConverter"/>-->
        <!--</list>-->
    <!--</property>-->
<!--</bean>-->
<mvc:annotation-driven>
    <mvc:message-converters>
        <bean class="org.springframework.http.converter.StringHttpMessageConverter" />
        <bean class="org.springframework.http.converter.json.MappingJackson2HttpMessageConverter" />
    </mvc:message-converters>
</mvc:annotation-driven>

Jackson依赖

<dependency>
    <groupId>com.fasterxml.jackson.core</groupId>
    <artifactId>jackson-core</artifactId>
    <version>${jackson.version}</version>
</dependency>
<dependency>
    <groupId>com.fasterxml.jackson.core</groupId>
    <artifactId>jackson-annotations</artifactId>
    <version>${jackson.version}</version>
</dependency>
<dependency>
    <groupId>com.fasterxml.jackson.core</groupId>
    <artifactId>jackson-databind</artifactId>
    <version>${jackson.version}</version>
</dependency>

Controller:

@RequestMapping(value = "/log.html", method = RequestMethod.POST)
    public ResponseEntity<LoginReply>  checkUser() {
        LoginReply loginReply = new LoginReply();
        loginReply.setTest("123");
        System.out.println("执行");
        return ResponseEntity.ok(loginReply);
    }

异步请求：

var data = 'username=' + $('#username').val() + "&";
data += 'password=' + $('#password').val();
    $.ajax('/user/log.html',{
        type: "POST",
        data: data,
        timeout: 100000,
        success: function (result) {
            alert(result);
        }
    });