Tweaked unbind call in once to ensure handler is removed

[DamonOehlman]

Hey Dmitry,

Just made a couple of tweaks to the once implementation to unbind the handler before triggering the handler. This means that in a case where the handler throws an exception, eve will still unbind the handler. Came across the behaviour while writing some tests that throw exceptions.

Cheers, Damon.

[DmitryBaranovskiy]

I can’t merge it, but I will merge it by hands.