Add Strict mode option

I suggest to add stric mode management option for connection. The pricipe of current version is to throw an exception when Response status code is 304. 
I think we could want to have a non-strict mode where no Exeption was throw. 

An use case is when you manage a subscription list, and someone already registred, ask for new subscription. The Mailjet API, will send a 304 code.

What is your opinion ?