Invisible properties present in Thing Descriptions

[benfrancis]

335 successfully stopped all properties being removed, but now invisible properties are slipping through.

STR:

• Install latest master of gateway
• Install latest master of zigbee-adapter
• Pair a coloured Zigbee light bulb

Expected:

• The coloured Zigbee lightbulb is added and has "On/Off" and "Color" properties

Actual:

• The coloured Zigbee lightbulb is added and has "On/Off", "Color" and "_level" properties

The _level property should have been filtered out by https://github.com/WebThingsIO/zigbee-adapter/blob/master/src/zb-classifier.js#L1934 and https://github.com/WebThingsIO/zigbee-adapter/blob/master/src/zb-node.js#L242