Refactor ModuleResult#pre_order

This is an optimization made by @danielkza at
805c7c1 which reduces the complexity of
finding the root finding it through the Processing.

I'm not sure the memozation through the attribute @pre_order worked at
and if we should keep it.

Author: rafamanzo

app/models/module_result.rb

@@ -31,9 +31,10 @@ def to_json(options={})
   end
 
   def pre_order
-    root = self
-    root = root.parent until root.parent.nil?
-    @pre_order ||= pre_order_traverse(root).to_a
+    return @pre_order unless @pre_order.nil?
+    return processing.root_module_result.pre_order unless parent.nil?
+
+    @pre_order = pre_order_traverse(self).to_a
   end
 
   def descendants

spec/models/module_result_spec.rb

@@ -164,11 +164,15 @@
       context 'when it is not the root module result' do
         let!(:root) { FactoryGirl.build(:module_result, id: 1)}
         let!(:child) { FactoryGirl.build(:module_result, id: 2)}
+        let!(:processing) { FactoryGirl.build(:processing, root_module_result: root)}
+
         before :each do
-          child.expects(:parent).twice.returns(root)
+          child.expects(:parent).returns(root)
+          child.expects(:processing).returns(processing)
           root.expects(:parent).returns(nil)
           root.expects(:children).returns([child])
         end
+
         it 'is expected to return the complete pre order tree traversal (from root)' do
           expect(child.pre_order).to eq([root, child])
         end