Fixed expiration, and cache-control: max-age implementation.

Also use `FILE.date` instead of `FILE.change_date` (`change_date` is the date of the last status change, quite often same as creation date, while `date` is the last modification date).

library/network/protocol/http/src/http_header_modifier.e

@@ -508,9 +508,16 @@ feature -- Others
 
 	put_expires (a_seconds: INTEGER)
 			-- Put "Expires" header to `a_seconds' seconds
+			-- and also "Cache-Control: max-age=.." .
+			-- To be supported by most browser.
+		local
+			dt: DATE_TIME
 		do
-			put_expires_string (a_seconds.out)
-		end
+			create dt.make_now_utc
+			dt.second_add (a_seconds)
+			put_expires_date (dt)
+			put_cache_control ("max-age=" + a_seconds.out)
+		end			
 
 	put_expires_string (a_expires: STRING)
 			-- Put "Expires" header with `a_expires' string value