done until  10.14,  join the model user and micropost

but has three problem :

1. the code below is deprecated , then how to use the keyword 'expect'
 @user.microposts.should == [newer_micropost, older_micropost]

 2. the code below not work in model micropost
   default_scope order: 'microposts.created_at DESC'

 3. todo : to replace the tab code into space