Fix unbinding events with object handlers

After the changes in https://github.com/rollbar/rollbar.js/commit/65a108e4e1b64bca6d6c6408849ff3c59260347f, event handlers implementing `EventListener` directly (i.e. plain non-function objects, https://developer.mozilla.org/en-US/docs/Web/API/EventListener) cannot not be unbound, since `Rollbar.prototype.wrap` passes them through without adding a `._wrapped` key.

This allows these types of handlers to be unbound. A more complete solution would also wrap them correctly :)
Merge pull request #33 from kmontag/patch-1

Fix unbinding events with object handlers