Improve Env constructor

lib/producer/core/env.rb

@@ -4,7 +4,7 @@ module Producer
       attr_reader   :current_recipe
       attr_accessor :target
 
-      def initialize(recipe)
+      def initialize(recipe = nil)
         @current_recipe = recipe
         @target         = nil
       end

spec/producer/core/env_spec.rb

@@ -2,20 +2,28 @@ require 'spec_helper'
 
 module Producer::Core
   describe Env do
-    let(:recipe)  { Recipe.new(proc { nil }) }
-    subject(:env) { Env.new(recipe) }
+    subject(:env) { Env.new }
 
     describe '#initialize' do
       it 'has no target' do
         expect(env.target).not_to be
       end
+
+      context 'without argument' do
+        it 'has no recipe' do
+          expect(env.current_recipe).not_to be
+        end
       end
 
-    describe '#current_recipe' do
-      it 'returns the assigned current recipe' do
+      context 'when a recipe is given as argument' do
+        let(:recipe)  { Recipe.new(proc { nil }) }
+        subject(:env) { Env.new(recipe) }
+
+        it 'assigns the current recipe' do
           expect(env.current_recipe).to eq recipe
         end
       end
+    end
 
     describe '#target' do
       let(:target) { Object.new }