Reset overflow counter on None onOverflow result

[ddworak]

Hey!
I know that according to the contribution guidelines I should've opened an issue, but this is just a short bugfix. I will be happy to do it in case you decide it's required here.
I believe that current behaviour of Drop*AndSignal wasn't intended and seems to have been introduced just recently. The docs state:

The function can return None in which case no message is sent and thus you can use it just to log a warning.

yet I've noticed that after an overflow the callback is called for each new message.
I've modified the tests to check for this behaviour and proposed a solution.

[alexandru]

Thanks a lot @ddworak, stupid bug, stupid test.

Thanks for catching and fixing it. Scala.js also has released a new version, I guess it is time for another minor release. After also fixing #247.

[alexandru]

@ddworak this is released in latest version 2.0.5