state always (!!!) is string -> this fixes individual on power flow card

fixes #483

Not sure, if this introduces other errors (again). Let's see... I did not yet see any.

lib/converters/genericConverter.js

@@ -48,8 +48,6 @@ exports.numericDeviceClasses = [
     'distance'
 ];
 
-const numericDeviceClassesIob = exports.numericDeviceClasses.concat(['timestamp']);
-
 exports.iobState2EntityState = function (entity, val, attribute) {
     let type = entity.context.type;
     const pos = type.lastIndexOf('.');
@@ -61,14 +59,17 @@ exports.iobState2EntityState = function (entity, val, attribute) {
         return val ? 'on' : 'off';
     } else if (type === 'binary_sensor') {
         return val ? 'on' : 'off';
-    } else if (typeof val === 'number' && entity.attributes && (numericDeviceClassesIob.includes(entity.attributes.device_class) || entity.attributes.unit_of_measurement)) {
-        return val; //do not convert timestamps or values that have a unit of measurement..
-    } else if (typeof val === 'number' && entity.attributes && ['date'].includes(entity.attributes.device_class)) {
-        const dateStr = new Date(val).toDateString(); //convert to date string
+    }else if (typeof val === 'number' && entity.attributes && ['date', 'timestamp'].includes(entity.attributes.device_class)) {
+        //convert to date string
+        const date = new Date(val);
+        let dateStr = date.toDateString();
+        if (attribute === 'timestamp') {
+            dateStr = date.toISOString();
+        }
         return dateStr === 'Invalid Date' ? 'unknown' : dateStr;
     } else if (type === 'lock') {
         return val ? 'unlocked' : 'locked';
-    } else if (typeof val === 'boolean' && type !== 'media_player') {
+    } else if (typeof val === 'boolean' && type !== 'media_player' && !attribute) { //attributes can have true/false.
         return val ? 'on' : 'off';
     } else if (typeof val === 'number' && entity.context.STATE.map2lovelace) {
         return entity.context.STATE.map2lovelace[val] || val;