Don't throw away data on user error.

If then() returns no data/undefined, don't throw the current data away.

Author: Sebmaster

lib/thread.js

@@ -192,7 +192,10 @@
 		var that = this;
 		var newOp = new Operation();
 		this.operation.then(function () {
-			that.data = cb(that.data);
+			var retData = cb(that.data);
+			if (retData !== undefined) {
+				that.data = retData;
+			}
 			newOp.resolve(null, that.data);
 		}, errCb);
 		this.operation = newOp;