[NICETHING beta] helpful whitespace classNameBindings assert

I've trolled myself many times thinking that 
classNameBindings works with a space separated string
(probably because of how bind-attr class="" works).
This should help me never troll myself again.

/cc @ebryn
Merge pull request #4217 from machty/classnamebindings-assert

[NICETHING beta] helpful whitespace classNameBindings assert