HTTP/2: improved handling of END_STREAM in a separate DATA frame. The save body filter saves the request body to disk once the buffer is full. Yet in HTTP/2 this might happen even if there is no need to save anything to disk, notably when content length is known and the END_STREAM flag is sent in a separate empty DATA frame. Workaround is to provide additional byte in the buffer, so saving the request body won't be triggered. This fixes unexpected request body disk buffering in HTTP/2 observed after the previous change when content length is known and the END_STREAM flag is sent in a separate empty DATA frame.