add test and fix bug for falsy connect key name

My check for whether you supplied the optional second argument for `Reflux.connect(listenable,key)` was too naïve. It uses a truth check and will therefore fail when you use a falsy key like `Reflux.connect(listenable,0)`. Which is a somewhat viable thing to do if you're doing array-like stuff in your render. 

This PR adds test and fixes the bug.