Added API for excluding flash messages, fixed typo in test

According to https://groups.google.com/forum/#!searchin/plataformatec-devise/flash/plataformatec-devise/IGmB_jvr3Ws/3MbPcsANmR4J you can already exclude specific flash messages by setting their value in `devise.en.yml` to either a blank string or `~`. This doesn't seem to be officially documented and my concern is that this behavior might change at some point.