Use HTTP.format_time method

src/kemal/init_handler.cr

@@ -1,3 +1,5 @@
+require "http"
+
 module Kemal
   # Initializes the context with default values, such as
   # *Content-Type* or *X-Powered-By* headers.
@@ -9,7 +11,7 @@ module Kemal
     def call(context : HTTP::Server::Context)
       context.response.headers.add "X-Powered-By", "Kemal" if Kemal.config.powered_by_header?
       context.response.content_type = "text/html" unless context.response.headers.has_key?("Content-Type")
-      context.response.headers.add "Date", Time.utc.to_rfc2822
+      context.response.headers.add "Date", HTTP.format_time(Time.utc)
       call_next context
     end
   end