Allow use another field as default

[Paulo Diovani Gonçalves]

Take the folowing example:

  split_accessor :starts_at, default: -> { Time.current }, time_format: '%H:%M'
  split_accessor :ends_at, default: -> { starts_at }, time_format: '%H:%M'

I want that, if ends_at is empty, TimeSplitter uses starts_at as default value.
Currently this is not allowed since the lambda is executed in Class scope.