put log to elasticsearch 6.3 got 406: "error":"Content-Type header [text/plain] is not supported

my config is

<appender name="ELASTIC" class="com.internetitem.logback.elasticsearch.ElasticsearchAppender">
        <url>http://myserver/_bulk</url>
        <index>log-%date{yyyy-MM-dd}</index>
        <type>tester</type>
        <loggerName>es-logger</loggerName> <!-- optional -->
        <errorLoggerName>es-error-logger</errorLoggerName> <!-- optional -->
        <connectTimeout>30000</connectTimeout> <!-- optional (in ms, default 30000) -->
        <errorsToStderr>false</errorsToStderr> <!-- optional (default false) -->
        <includeCallerData>false</includeCallerData> <!-- optional (default false) -->
        <logsToStderr>false</logsToStderr> <!-- optional (default false) -->
        <maxQueueSize>104857600</maxQueueSize> <!-- optional (default 104857600) -->
        <maxRetries>3</maxRetries> <!-- optional (default 3) -->
        <readTimeout>30000</readTimeout> <!-- optional (in ms, default 30000) -->
        <sleepTime>250</sleepTime> <!-- optional (in ms, default 250) -->
        <rawJsonMessage>false</rawJsonMessage> <!-- optional (default false) -->
        <includeMdc>false</includeMdc> <!-- optional (default false) -->
        <maxMessageSize>100</maxMessageSize> <!-- optional (default -1 -->
        <authentication class="com.internetitem.logback.elasticsearch.config.BasicAuthentication" /> <!-- optional -->
        <properties>
            <property>
                <name>host</name>
                <value>${HOSTNAME}</value>
                <allowEmpty>false</allowEmpty>
            </property>
            <property>
                <name>severity</name>
                <value>%level</value>
            </property>
            <property>
                <name>thread</name>
                <value>%thread</value>
            </property>
            <property>
                <name>stacktrace</name>
                <value>%ex</value>
            </property>
            <property>
                <name>logger</name>
                <value>%logger</value>
            </property>
        </properties>
        <headers>
            <header>
                <name>Content-Type</name>
                <value>text/plain</value>
            </header>
        </headers>
    </appender>

internetitem / logback-elasticsearch-appender

put log to elasticsearch 6.3 got 406: "error":"Content-Type header [text/plain] is not supported #53