[Feature] Headers [Differences from 3.5 SP1] 
The following improvements were made to headers: WCF Data Services now correctly rejects an eTag header that has an unspecified value. WCF Data Services now returns an error and does not execute the request for a delete request to a link when an if-* header is in the request. WCF Data Services now returns an error to the client in the format (Atom, JSON) that the client specified in the Accept header.