Included StoreAsync methods to IAsyncDocumentSession

Apparently I let slip some whitespace change to Generate.cs, probably screwed when rebasing or something. Can you guys ignore this?

Also on Raven.Client.Lightweight/Extensions/AsyncDocumentSessionExtesions.cs line 593. I've had included a throw InvalidOperationException("couldn't generate Id for entity") when the task.Result was null but removed it later.

Should I had included it again?