Fix various issues that could cause the callback to `onDocumentReady` to
fire after the subscription had been canceled.

 - `pollOnUnload` was called twice for the initial document, once at the
   top level of the function and once by the first call to
   `checkForDocumentChange`. This resulted into "unload" listeners being
   added for the same window.

 - The initial async `checkForDocumentChange` call would still fire if
   the subscription was immediately canceled

 - If the current window's "unload" event fired after the subscription
   was canceled, polling would be restarted, effectively re-subscribing
   to document changes.