bugfix: conference verb will have '_' property when leg is moved from other FS

lib/tasks/task.js

@@ -336,6 +336,9 @@ class Task extends Emitter {
         }
         required = required.filter((item) => item !== dKey);
       }
+      else if (dKey === '_') {
+        /* no op: allow arbitrary info to be carried here, used by conference e.g in transfer */
+      }
       else throw new Error(`${name}: unknown property ${dKey}`);
     }
     if (required.length > 0) throw new Error(`${name}: missing value for ${required}`);