Refactor working in uuid method test for ModelDecorator
Red:  ModelDecorator#decorate adds a uuid to a model

-----FAILING TEST-----

1) ModelDecorator#decorate adds a uuid to a model
   Failure/Error: uuid               = ModelDecorator.new.decorate(model_without_uuid)
   ArgumentError:
     wrong number of arguments (0 for 1)
Modify ModelDecorator#initialize so that model default is nil

-----FAILING TEST-----

1) ModelDecorator#decorate adds a uuid to a model
   Failure/Error: uuid               = ModelDecorator.new.decorate(model_without_uuid)
   NoMethodError:
     undefined method `decorate' for #<ModelDecorator:0x007fc27b34b3f0 @model=nil>