add destroy attachment test

spec/acceptance/learning_objects_spec.rb

@@ -183,6 +183,22 @@ resource 'Learning Objects' do
     end
   end
 
+  delete '/v1/learning_objects/:learning_object_id/attachment/:attachment_id' do
+    include_context "authenticate_user"
+
+    let(:learning_object_id) { @learning_object.id }
+    let(:attachment_id) { @learning_object.attachments.first.id  }
+
+    before do
+      @learning_object = create(:learning_object, publisher: @user)
+      @learning_object.attachments << LearningObject::Attachment.create
+    end
+
+    example 'Destroy an attachment (file) of a learning object' do
+      do_request
+      expect(status).to eq(200)
+    end
+  end
 
 
 end