ApiReview: Make most properties settable on HttpContext, HttpRequest, HttpResponse, etc.

[Tratcher]

Anything that makes sense to set during testing.

Yes:
Items (& IItemsFeature.Items)
RequestAborted
Request.Cookies (& IRequestCookiesFeature.Cookies)
Request.IsHttps
Request.Query (& IQueryFeature.Query)

Maybe:
Request, Response
Request.Headers
Response.Headers
Response.HeadersSent
Response.Cookies - not a collection

No:
IsWebSocketRequest (You have to replace the entire feature for testing)
WebSocketRequestedProtocols
Request.HasFormContentType (set the Form property instead)

[Tratcher]

Remove DefaultHttpContext.Revision.

[Tratcher]

HttpContext.Session can't be easily set. The property is an ISessionCollection, but the feature stores an ISession that the collection wraps. We may need to think about the layering there.