Allow defaults with lambdas of arity 0 to be overriden

[jcmfernandes]

Hi Rodrigo! 👋

Let me start by thanking you for your work on the "micro" family of gems. Great stuff!

I stumped across this bug earlier today. I couldn't override a default when playing around with u-case, and I had to understand why. Here is why.

As a more general remark, the default optional seems somewhat overloaded. Maybe we could move the value preprocessing behavior to a different option. What do you think? It would be a breaking change.

Obrigado! 🏆

[jweslley]

@serradura I agree with the changes in this PR. I started using u-attrs recently and I didn't expect the actual behavior. I'm not able to override the default value when passing the attribute to initialize.

For now, I'm using attribute :name, default: -> name { name || 'default value' } as a workaround, but it is more verbose than just attribute :name, default: -> { 'default value' }