API: fixed double decoration

Decorators should implement `Magic::Decoratable` API not to fail upon an accidental double decoration. They shouldn't be in fact decorated though.

CHANGELOG.md

@@ -5,6 +5,9 @@
 - Improved extendability: one may override `Magic::Decoratable#decorator_base` to be used for lookups.
 - `Magic::Decoratable.classes` for all the decoratables.
 
+## Fixed
+
+- Failures on double decoration attempts.
 
 ## [0.2.0] — 2024-10-17
 

lib/magic/decorator/base.rb

@@ -5,7 +5,8 @@
 module Magic
 	module Decorator
 		class Base < SimpleDelegator
-			extend Lookup
+			extend  Lookup
+			include Decoratable
 
 			class << self
 				def name_for(object_class) = "#{object_class}Decorator"

spec/magic/decorator/base_spec.rb

@@ -110,6 +110,12 @@ def in?(collection) = collection.include? self
 					end
 				end
 			end
+
+			context 'when double-decorated' do
+				subject { decorated.decorate }
+
+				it { is_expected.to be_nil }
+			end
 		end
 	end
 end