Fixes "TypeError: Cannot set property '3' of undefined"

`dps` does not exist yet so we cannot set a property of undefined.

('3' is the brightness for Tuya light bulbs)

Author: dresende

index.js

@@ -230,7 +230,7 @@ TuyaDevice.prototype.set = function (options) {
   if (options.dps === undefined) {
     thisRequest.dps = {1: options.set};
   } else {
-    thisRequest.dps[options.dps.toString()] = options.set;
+    thisRequest.dps = {[options.dps.toString()]: options.set};
   }
 
   debug('Payload: ');