Allow resetting defaults

[mustmodify]

class TravelEstimationService < Valuable
  has_collection :resources
end

if CONFIG[:live]    
  TravelEstimationService.set_default(:resources => [ MappingProvider::RouteInfo ]
end

[mustmodify]

Another example:

class GenericInterface < Valuable
  has_value :encoding, default: 'UTF-8'
end

class ActualInterface < GenericInterface
  set_default encoding: 'ISO-8859-1'
end