Refactor HTTP response builder

api/category.go

api/entry.go

api/feed.go

api/icon.go

api/opml.go

api/subscription.go

api/user.go

daemon/routes.go

fever/fever.go

http/response/builder.go

http/response/builder_test.go

http/response/doc.go

http/response/html/doc.go

http/response/html/html.go

http/response/html/html_test.go

http/response/json/doc.go

http/response/json/json.go

http/response/json/json_test.go

http/response/response.go

http/response/xml/doc.go