Implement `set?' task keyword:

When called from a task, will return true when given key exists and
false when key does not exist.

features/registry.feature

@@ -56,3 +56,16 @@ Feature: key/value registry
       """
     When I successfully execute the recipe
     Then the output must contain "some_value"
+
+  Scenario: `set?' keyword tests wether given key is defined
+    Given a recipe with:
+      """
+      set :some_key, 'some_value'
+      task :registry_testing do
+        echo 'some_value_set' if set? :some_key
+        echo 'other_value' if set? :other_key
+      end
+      """
+    When I execute the recipe
+    Then the output must contain "some_value_set"
+    And the output must not contain "other_value"

lib/producer/core/env.rb

@@ -2,7 +2,7 @@ module Producer
   module Core
     class Env
       extend Forwardable
-      def_delegators :@registry, :[]=
+      def_delegators :@registry, :[]=, :key?
 
       attr_reader   :input, :output, :error_output, :registry, :logger
       attr_accessor :target, :verbose, :debug, :dry_run

lib/producer/core/task.rb

@@ -17,6 +17,7 @@ module Producer
       def_delegators  :@name, :to_s
       def_delegators  :@env,  :target
       def_delegator   :@env,  :[], :get
+      def_delegator   :@env,  :key?, :set?
 
       define_action :echo,                  Actions::Echo
       define_action :sh,                    Actions::ShellCommand

spec/producer/core/task_spec.rb

@@ -160,6 +160,18 @@ module Producer::Core
       end
     end
 
+    describe '#set?' do
+      before { env[:some_key] = :some_value }
+
+      it 'returns true when given key exists' do
+        expect(task.set? :some_key).to be true
+      end
+
+      it 'returns false when given key does not exist' do
+        expect(task.set? :other_key).to be false
+      end
+    end
+
     describe '#target' do
       before { env.target = :some_target }