> Concatenation does not make sense with an unordered collection.

There are also the merge and delete methods. Should + be simply
aliases for those? I personally would favour making those operators
work on the set of keys.

> > Any objections to nil.to_hash?

Nil is nil. Maybe there is a reason why the variable/value in question
is nil and not {}? If so, you probably shouldn't ignore that
distinction?