fix: improve event prop merging; add currentTarget

[ryo-fujinone]

fix #112

In some events, mergeObjects() fails to copy certain properties

When merging progress and abort events, several errors occur (probably also with error events and timeout events).
* Since there is a try-catch, xhook does not stop and no error is output.

TypeError: Cannot set property currentTarget of #<Event> which has only a getter
TypeError: Cannot assign to read only property 'CAPTURING_PHASE' of object '#<Event>'

This can be fixed by using Object.defineProperty().

currentTarget is missing in all events

As mentioned above, mergeObjects() fails to copy currentTarget.
In addition, events dispatched by xhook with empty objects do not include currentTarget.
In other words, currentTarget is missing from every event.
This can be fixed by using Object.defineProperty() for events where currentTarget is null.

* I intentionally left out srcElement because it’s a deprecated alias for target.