Implement -respondsToSelector: when using -rac_signalForSelector: on KVO'd target

Fixes #889 

When calling `-rac_signalForSelector:` on an object that has been KVO'd, we implement the selector directly on KVO's dynamic subclass. However, the default implementation of `-respondsToSelector:` bases its result on the class returned by `-class`, which KVO overrides to return the original class, not its custom subclass.

This change redefines `-respondsToSelector:` when calling `-rac_signalForSelector:` on a target that has been KVO subclassed.