fix(shell_spec): Clear TODO list before testing

In other tests we add todos and this is a cached singleton
attribute.

Author: tvdeyen

spec/libraries/shell_spec.rb

@@ -10,7 +10,10 @@ class MyToDoList
   end
 
   describe Shell do
-    before { allow(MyToDoList).to receive(:puts) }
+    before do
+      MyToDoList.todos.clear
+      allow(MyToDoList).to receive(:puts)
+    end
 
     describe ".todo" do
       it "should add given string as a todo by delegating to .add_todo" do