add response header Date

[mrchypark]

https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Date

Date res header provide is helpful.

In my case, I use middleware like below.

  process_response = function(.req, .res) {
    # compress body
    .res$set_header("Date", paste(format(Sys.time(), "%a, %d %b %Y %H:%M:%S", tz = "gmt"), "GMT"))
  },
  id = "date_header"
)```

but may add default setting, It will usefull.

[artemklevtsov]

RestRserve::Response provides the set_date method for this.