Merge: Counter: fixes

Hey there,

Some fixes to counter:
* the first one should not be subject to discussion since it only fix the to_s nullable elements.
* the second one is more tricky. I propose to change the add_all behavior to increment all values instead of just rewriting them.

@privat, you are the main user of the `counter` lib, what is you feeling about that?

Pull-Request: #2377