Added connection header related functions.

- WSF_REQUEST.is_keep_alive_http_connection: BOOLEAN
  - HTTP_HEADER_MODIFIER.put_connection_keep_alive
  - HTTP_HEADER_MODIFIER.put_connection_close
In Standalone request handler code, better detection of Connection: keep-alive header.

library/server/wsf_proxy/reverse_proxy/wsf_simple_reverse_proxy_handler.e

@@ -291,11 +291,8 @@ feature {NONE} -- Implementation
 			response.set_status_code (a_status_code)
 			response.header.put_content_type_text_html
 			response.header.put_content_length (s.count)
-			if
-				attached request.http_connection as l_connection and then
-				l_connection.is_case_insensitive_equal_general ("keep-alive")
-			then
-				response.header.put_header_key_value ("Connection", "keep-alive")
+			if request.is_keep_alive_http_connection then
+				response.header.put_connection_keep_alive
 			end
 			response.put_string (s)
 		end