[v2] Async doesn't accept filterOption

[nadeemkhedr]

Not sure if its a bug or its a design decision, Async component in the proptypes expects filterOption but when it renders < SelectComponent> it always set it to null

react-select/src/Async.js

Line 170 in f1cb4ee

filterOption={null}

If its a design decision to remove it, I can share why we do need it in our project, currently this is the only thing that is blocking me from upgrading

[JedWatson]

This was a specific decision, but I've realised that simply defaulting the prop would cover the same default behaviour I wanted while unblocking you - so I've implemented that in #2822