Consider returning the default value instead of `nil`, when sending `:first` or `:last` to an empty collection

[stormwatch]

It is my understanding that all these sorted collections rely ultimately on the behaviour of sorted array, which right now is returning nil always for an empty collection, regardless of the default configured value when the collection is instantiated. Besides of returning the default value, also consider adding first_if_empty(&block_closure) and :last_if_empty(&block_closure) and/or allowing to optionally pass an a block to :first and :last in the same manner as :fetch does.

By the way, thanks for publicly sharing this collection of collections in the first place. All of these should be included in the standard library.

[GarrisonJ]

@stormwatch That's an interesting idea.

It makes sense, but I'd need to weight it against the simplicity of keeping the API between the three containers the same. I'd want to see more interest or need for this change before moving forward.

If there is a situation where Hash returns the default value, but SortedHash doesn't, I would consider that a bug, as my goal is to match Hash's API as closely as possible.

Thanks for your insights and for the kind words about the library. I appreciate your support and feedback!