storage/http: specify default behavior upon nil Roundtripper
storage/http: specify default behavior upon nil Roundtripper

Signed-off-by: Luca Bruno <54b63133bb17564df145c2c274f1bc31eb9aa743@coreos.com>

HuKeping/notary,johnsandiford/notary,docker/notary,vdemeester/notary,jfrazelle/notary,johnsandiford/notary,docker/notary,docker/notary,jfrazelle/notary,johnsandiford/notary,endophage/notary,HuKeping/notary,vdemeester/notary,jfrazelle/notary,vdemeester/notary,endophage/notary,HuKeping/notary,endophage/notary