odata.isolation should be a header rather than an expectation

ODATA-317 added support for snapshot isolation by specifying a new odata.isolation expect header. However, upon further research, expect headers must be understood by intermediaries or rejected. so even though a service might support snapshot isolation, intermediaries would likely return expectation failed.

We discussed making this a preference, but earlier we had said that the desired behavior was for the service to fail the request if the client specified snapshot and the service was unable to provide that level of isolation, and preferences should not cause errors.

So it seems the best way to support this functionality is to introduce a new odata header for isolation.