Fix Humidity Sensor

Prior to this commit the humidity sensor urn was incorrect and caused
zero to be returned. This commit fixes both that and the variable name
to be correct per the [mios
wiki](http://wiki.mios.com/index.php/Luup_UPNP_Files#HumiditySensor1)

Tested with the [Everspring Combo
Device](http://www.amazon.com/gp/product/B0078FD8FY)