Added isIncludeContext check to access logger encoder.

[kersten]

This PR adds the same functionality to the access logger that is already implemented in the standard JSON logger.

If one set the element includeContext to false the context of the logger is not added to the JSON output.

<configuration>
    <!-- always a good activate OnConsoleStatusListener -->
    <statusListener class="ch.qos.logback.core.status.OnConsoleStatusListener"/>
    <appender name="STDOUT" class="ch.qos.logback.core.ConsoleAppender">
        <encoder class="net.logstash.logback.encoder.LogstashAccessEncoder">
            <timeZone>UTC</timeZone>
            <includeContext>false</includeContext>
            <fieldNames>
                <requestHeaders>request_headers</requestHeaders>
                <responseHeaders>response_headers</responseHeaders>
            </fieldNames>
        </encoder>
    </appender>
    <appender-ref ref="STDOUT"/>
</configuration>

[philsttr]

Thanks!!