Expose Status as a Response extension

[tamasfe]

Add the Status itself as an extension to a http::Response so that it is available in middleware.

Motivation

I'd like to implement service-level error inspection. Status contains a source that allows returning pretty much any error, however this is lost when it is converted into a response. The only way to pass information to middleware is headers which is expensive and requires serializable types.

Solution

Simply added the status to the response extensions in Status::into_http(). How a Status is turned into a Response is not specified in the documentation so I did not add additional notes regarding this feature either.