Fix HEAD request response when request not given to response.

If a middleware (swift3, I'm looking at you), doesn't pass a Request
object into the Response constructor, Response._response_iter cannot
know to send zero bytes in the body of the HEAD response.

This patch fixes this usage of swob by making Response.__call__
helpfully reify self.request from env if it wasn't already set by the
Response object's constructor.

This fixes a bug in swift3 + swob-enabled-Swift where HEAD requests to
swift3 resulted in a response with a body in violation of the relevant
RFC and confusing clients.

Thanks to kostecky for finding the bug and describing it accurately.

Change-Id: I2bdb098052b161e1cddf1e4e482ab4dfafeb18c0
Merge "Fix HEAD request response when request not given to response."