Mappings are not correctly handled in the JSON Collection Handler

The Mapping values mechanism is not correctly handled in the JSON Collection Handler, the mapped value is always NaN.

How to easily reproduce (accessing to Zoom Service Status for current status):
```
collectd-configuration.xml:

    <service name="Zoom-Status-Service" interval="10000" user-defined="false" status="on">
         <parameter key="collection" value="zoom-stats"/>
         <parameter key="handler-class" value="org.opennms.protocols.json.collector.DefaultJsonCollectionHandler"/>
      </service>

   <collector service="Zoom-Status-Service" class-name="org.opennms.protocols.xml.collector.XmlCollector"/>

xml-datacollection-config.xml:

    <xml-collection name="zoom-stats">
        <rrd step="10">
            <rra>RRA:AVERAGE:0.5:1:60480</rra>
            <rra>RRA:AVERAGE:0.5:360:1488</rra>
            <rra>RRA:AVERAGE:0.5:8640:366</rra>
            <rra>RRA:MAX:0.5:8640:366</rra>
            <rra>RRA:MIN:0.5:8640:366</rra>
        </rrd>
      <xml-source url="https://status.zoom.us/api/v2/summary.json">
            <xml-group name="zoom-status" resource-type="zoomStatus" resource-xpath="/components" key-xpath="@name" >
                <xml-object name="componentName" type="string" xpath="@name" />
                <xml-object name="componentStatus" type="string" xpath="status" />
                <xml-object name="status" type="GAUGE" xpath="status" >
                    <xml-mapping from="operational" to="5555" />
                    <xml-mapping from="scheduled" to="6666" />
                </xml-object>
            </xml-group>
        </xml-source>
     </xml-collection>
```