Fixed a couple minor issues I noticed when adding authenticators

* I believe klass should be *args, in the method add_authenticator on EM::Imap.
* If you specify an authenticator as a symbol (as in the sample code for gmail_xoauth), it converts the authenticator to an uppercase symbol `:foo` I think the correct behavior is to convert this to an uppercase string FOO.

I added specs around both these bugs. Let me know if there's anything I can change. Also, it would be awesome if you'd ping me when you've published a new version of the gem, so that I can get off my fork :+1: