Delegate to private methods

Issue #526
Previously if a class delegated to a private method the `matches?` method would
return `false`. This was because`respond_to?` always returns `false` for private methods, this commit fixes this by making `respond_to?` evaluate private methods.

lib/shoulda/matchers/independent/delegate_matcher.rb

@@ -139,7 +139,7 @@ def subject_has_delegating_method?
         end
 
         def subject_has_target_method?
-          subject.respond_to?(target_method)
+          subject.respond_to?(target_method, true)
         end
 
         def ensure_target_method_is_present!

spec/shoulda/matchers/independent/delegate_matcher_spec.rb

@@ -131,6 +131,41 @@ def mailman
     end
   end
 
+  context 'given a private method that delegates properly' do
+    before do
+      define_class(:mailman) do
+        def deliver_mail
+        end
+      end
+
+      define_class(:post_office) do
+        def deliver_mail
+          mailman.deliver_mail
+        end
+
+        def mailman
+          Mailman.new
+        end
+
+        private :mailman
+      end
+    end
+
+    it 'accepts' do
+      post_office = PostOffice.new
+      expect(post_office).to delegate_method(:deliver_mail).to(:mailman)
+    end
+
+    it 'produces the correct failure message if the assertion was negated' do
+      post_office = PostOffice.new
+      message = 'Expected PostOffice#deliver_mail not to delegate to PostOffice#mailman, but it did'
+
+      expect {
+        expect(post_office).not_to delegate_method(:deliver_mail).to(:mailman)
+      }.to fail_with_message(message)
+    end
+  end
+
   context 'given a method that delegates properly with arguments' do
     before do
       define_class(:mailman)