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.

examples/simple_ssl/application_execution.e

@@ -32,8 +32,8 @@ feature -- Basic operations
 			response.set_status_code ({HTTP_STATUS_CODE}.ok)
 			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