Fix KeyError while generating a WSGI response
Fix KeyError while generating a WSGI response

The KeyError exception raised while generating a WSGI response based on
the 413 exception. Because there may be no "Retry-After" attribute in the
exception headers.

Change-Id: I89eb1878acc2195775be17fbcba8088f2e00bac1
Closes-Bug: #1259867

petrutlucian94/cinder,CloudServer/cinder,potsmaster/cinder,rakeshmi/cinder,mahak/cinder,NetApp/cinder,Accelerite/cinder,redhat-openstack/cinder,Thingee/cinder,nexusriot/cinder,tlakshman26/cinder-bug-fix-volume-conversion-full,bswartz/cinder,takeshineshiro/cinder,Nexenta/cinder,scality/cinder,abusse/cinder,phenoxim/cinder,saeki-masaki/cinder,bswartz/cinder,rakeshmi/cinder,Akrog/cinder,winndows/cinder,Akrog/cinder,hguemar/cinder,cloudbase/cinder,Hybrid-Cloud/cinder,apporc/cinder,JioCloud/cinder,Datera/cinder,nikesh-mahalka/cinder,NetApp/cinder,potsmaster/cinder,Thingee/cinder,blueboxgroup/cinder,julianwang/cinder,openstack/cinder,NeCTAR-RC/cinder,Nexenta/cinder,petrutlucian94/cinder,tlakshman26/cinder-bug-fix-volume-conversion-full,tlakshman26/cinder-new-branch,spring-week-topos/cinder-week,Accelerite/cinder,cloudbase/cinder,scality/cinder,alex8866/cinder,scottdangelo/RemoveVolumeMangerLocks,nikesh-mahalka/cinder,JioCloud/cinder,hguemar/cinder,ge0rgi/cinder,spring-week-topos/cinder-week,alex8866/cinder,takeshineshiro/cinder,Thingee/cinder,phenoxim/cinder,openstack/cinder,sasukeh/cinder,duhzecca/cinder,manojhirway/ExistingImagesOnNFS,Hybrid-Cloud/cinder,redhat-openstack/cinder,duhzecca/cinder,github-borat/cinder,j-griffith/cinder,eharney/cinder,Paul-Ezell/cinder-1,leilihh/cinder,scottdangelo/RemoveVolumeMangerLocks,tlakshman26/cinder-https-changes,apporc/cinder,leilihh/cinder,j-griffith/cinder,abusse/cinder,eharney/cinder,winndows/cinder,tlakshman26/cinder-https-changes,tlakshman26/cinder-new-branch,sasukeh/cinder,dims/cinder,julianwang/cinder,Paul-Ezell/cinder-1,blueboxgroup/cinder,mahak/cinder,github-borat/cinder,CloudServer/cinder,saeki-masaki/cinder,Datera/cinder,dims/cinder,nexusriot/cinder,manojhirway/ExistingImagesOnNFS,NeCTAR-RC/cinder