Added missing test cases for `post.author`
Added missing test cases for `post.author`

no issue

- Ghost does not support adding an author by relation (`post.author = {id: '..'}`)
- Ghost does not support editing an author by relation (`post.author = {id: '..'}`)
- only `author_id` is allowed

Kikobeats/Ghost,acburdine/Ghost,Kikobeats/Ghost,kevinansfield/Ghost,dbalders/Ghost,JohnONolan/Ghost,chevex/undoctrinate,TryGhost/Ghost,ErisDS/Ghost,kevinansfield/Ghost,acburdine/Ghost,e10/Ghost,TryGhost/Ghost,Gargol/Ghost,TryGhost/Ghost,bitjson/Ghost,e10/Ghost,JohnONolan/Ghost,acburdine/Ghost,janvt/Ghost,dbalders/Ghost,Gargol/Ghost,ErisDS/Ghost,dbalders/Ghost,Kikobeats/Ghost,JohnONolan/Ghost,bitjson/Ghost,janvt/Ghost,kevinansfield/Ghost,ErisDS/Ghost