Iterators\Filter: count() counts only fitlered items

Fixes behaviour of count() method - now it counts only filtered items, not all items
Not sure that I understand correctly what you mean. It cannot behave exactly like CallbackFilterIterator, as it has the same problem, which I want to fix.

Really don't no how could I remove the condition (if you don't wanna count method used in previous solution). There must be the fix of count functionality and there must be proxy for additional functionality of filtered iterator.