I was playing on my test world for 1.8 and started with the WitherBoss and Spectator Mode. As I was testing I noticed that the WitherBoss was shooting me after I was in survival, but not before that. It wasn't coming towards me. It seemed to be in Idle.
Incomplete
Your title indicates that you are getting attacked by a Wither in Spectator Mode, while your description says the complete opposite. Please provide more detailed information.