Observable.ignoreElements - optimize

[davidmoten]

As suggested by @akarnokd, instead of filter(alwaysFalse) use a custom operator and a singleton instance of it.

Also snuck in the removal of an unused import from OperatorTakeLastOneTest.java.

[akarnokd]

This will forward the child's requests and will hang because the child doesn't receive an element and nobody is requesting more. I'd instead write this:

Subscriber<T> result = new Subscribeer<T>() { ... }
child.add(result);
return result;

[davidmoten]

Ah thanks. Now I understand the difference between those two approaches to connecting the subscribers. Simple really. I might add that bit of info to the javadoc on the Subscriber constructor I used in another PR, I was thinking of adding bit more doco to Subscriber anyway because it gets stepped through a lot when you debug and it's a bit terse (confusing variable names and stuff).

[davidmoten]

changes made including a unit test of the request problem (doesn't hang by the way, just terminates early, without processing all upstream).

[akarnokd]

Great, thanks!