Use HTTPName for cookies in response encoders

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

The upgrade to v3.12.1 in the cookie example shows how the cookie named
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 }},