Doc::destroy: fix call to this inside error case of async callback

Author: nateps

lib/client/doc.js

@@ -107,9 +107,8 @@ Doc.prototype.destroy = function(callback) {
     if (doc.wantSubscribe) {
       doc.unsubscribe(function(err) {
         if (err) {
-          if (callback) callback(err);
-          else this.emit('error', err);
-          return;
+          if (callback) return callback(err);
+          return doc.emit('error', err);
         }
         doc.connection._destroyDoc(doc);
         if (callback) callback();