Harmonized the null of Maybe.IEnumerable with the rest of monads.net

Hi there.

I was thinking abit over how monads.net works for objects and IEnumerable. IMHO in my mind it makes more sense if the extension methods for IEnumerable check each object in the collection for null and applies the action if not null, otherwise skip.

This makes for more null-safe code as well as avoiding irritating null reference exceptions and IMHO is more in spirit with the rest of monads.net.

Of course you are free to disagree with all of this and deny the pullrequest.

.With with key for IDictionary is a gem!

Best regards,
Mårten