Use HTTPName for cookies in response encoders

I believe this was an oversight in 27c249f (included in v3.12.0 and newer),
breaking cookie name mapping to some extent.

The upgrade to v3.12.1 in the cookie example shows how the name assigned to the
cookie inadequately changes from `SID` (cookie name) to session_id` (attr name).

goadesign/examples@45ca835#diff-c31b43a7b7656de4c0d63b7ac526bfbad82aa335c79f08e186f5d78c6ddc0802R97

http/codegen/server.go

@@ -1335,7 +1335,7 @@ const responseT = `{{ define "response" -}}
 		{{ .VarName }} := "{{ printValue .Type .DefaultValue }}"
 		{{- end }}
 		http.SetCookie(w, &http.Cookie{
-			Name: {{ printf "%q" .Name }},
+			Name: {{ printf "%q" .HTTPName }},
 			Value: {{ .VarName }},
 			{{- if .MaxAge }}
 			MaxAge: {{ .MaxAge }},