Deprecated, use `user_agent.original` instead.
@example CERN-LineMode/2.15 libwww/2.17b3 @example Mozilla/5.0 (iPhone; CPU iPhone OS 14_7_1 like Mac OS X) AppleWebKit/605.1.15 (KHTML, like Gecko) Version/14.1.2 Mobile/15E148 Safari/604.1
Replaced by `user_agent.original`.
public const HTTP_USER_AGENT = 'http.user_agent';