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."
get swob.Request.blank to parse path

Change-Id: I3327c915b3b868bb1829103adb718632e58b1b4a
Merge branch 'master' into rackspace_production
Merge branch 'swob_refactor' into rackspace_production
Changelog for 1.7.6~rax9

Change-Id: I0fbc0494d9fa15e077fc44836fdeccd2b9bf2b3d