Accept an option 'default' to allow setting default serialized value for association if null

[vinaya-procore]

Included in the PR

For models that have belongs_to association that allow null relationship, blueprinter serializes to null by default. This PR adds an ability to add a default option to blueprinter association which can be used as the serialized value instead of null.

In cases when the association company is nil, the behavior looks as follows:

Before

class UserBlueprint < Blueprinter::Base
    identifier :uuid
    association :company, blueprint: CompanyBlueprint
end

UserBlueprint.render(user) renders { "id": 1, "company": null }

After

class UserBlueprint < Blueprinter::Base
    identifier :uuid
    association :company, blueprint: CompanyBlueprint, default: {}
end

UserBlueprint.render(user) renders { "id": 1, "company": {} }

[philipqnguyen]

Looks good to me! Since it's a new feature, would you also bump version.rb to 0.5.0 and update the changelog.md as well?