Add default option to vault_attribute

[justincampbell]

This adds a :default option to vault_attribute, allowing a default value to be set if the underlying value is nil.

vault_attribute :metadata,
  serializer: :json,
  default: {}

vault_attribute :access_level,
  default: "readonly"

This enables a simple fix for applications affected by the breaking change introduced in #81 for serialize: :json empty values.

[findkim]

This is a neat option!

Would this mean if someone had used this library prior to #81 it would require setting the default to {} when updating the dependency or do a migration if it had relied on the nil or "" to decode as {}?

[justincampbell]

@findkim yup! will call that out in the changelog