Add ability to retrieve HTTP status code from HttpResponse

The use case here is to be able to get the response status code
in a backend-agnostic way, i.e. without resorting to downcasts of
the underlying response object. This is useful when writing very
generic intents, e.g. for doing metrics on the request/response
cycle.

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See also https://groups.google.com/d/msg/unfiltered-scala/m1Tu0rqtTwo/R7Ij3YDW6j0J