We added a "safety net" in swob that won't let HEAD responses return a
body. It seems like we don't currently have many unittests that both
enforce leak tracking and ALSO make HEAD requests. The way swob works
currently it's not possible for even a "well behaved client" to get a
clean assert with FakeSwift's leak tracking because the leak tracking
iterator isn't returned to the client to close and swob doesn't do
anything to close down and consume the HEAD response iterator it's
throwing away.
RelatedChange: I168e147aae7c1728e7e3fdabb7fba6f2d747d937
Change-Id: I079315ebeb772eaad1bf190fefd132b4087b4897
9e065e2d23