PublishOptions type issue fix

When using Ably with TypeScript, I cannot build because I am told that quickAck cannot be undefined based on the definition below, yet quickAck has a `?` which I assume means it does allow  boolean or undefined as a value. This is an incompatible change introduced in 02de82b and is breaking TypeScript builds.

Author: mattheworiordan

ably.d.ts

@@ -2287,7 +2287,7 @@ export type PublishOptions = {
    * Support any publish options that may be added serverside without needing
    * typings changes.
    */
-  [k: string]: string | number | boolean;
+  [k: string]: string | number | boolean | undefined;
 };
 
 /**