Subscription callback fixes

I found further problems in the SubscriptionDefinition, this time I wanted to contribute myself.

My focus was on ensuring the context is always set for modified callbacks.
This pull request contains the tests and the corresponding fixes.

I also have identified further issues, for which I want to clarify:
* subscribe() resets any previously selected callback strategy (delayed or limited) - is this the intent? I'd guess these should stick and subscribe() should only modify the user-callback .
* Both types of strategies can be chained endless right now. I don't think it makes sense and would rather understand that for instance withDelay(x) and later on withDelay(y) would only have it use y instead of x+y . Would you agree on having just one limiting strategy (either throttle or debounce) and just one delay strategy (either defer or withDelay)? Similar with once/disposeAfter.
* A self-disposing callback could still fire if it is delayed and triggered several times before the first one of the batch unregistered the subscription. This should also be irrelevant to order of configuration (disposeAfter().withDelay() or withDelay().disposeAfter() ) and prevent late firing of unsubscribed callbacks.

If any/all of these issues are to be handled, how may I proceed?
* Append to this pull request or
* Create dedicated branches/pull requests per topic?

(Note: I also fixed the relative paths in the test-html for standalone tests)