canonical_string refactoring + fix typing in middleware comment
canonical_string refactoring + fix typing in middleware comment

stackforge/swift3,KoreaCloudObjectStorage/swift3,KoreaCloudObjectStorage/swift3,swiftstack/swift3-stackforge,citrix-openstack-build/swift3,citrix-openstack/build-swift3,stackforge/swift3,citrix-openstack/build-swift3,tumf/swift3,citrix-openstack-build/swift3,tumf/swift3,swiftstack/swift3-stackforge