src/Tweeper.php: make enclosure validate when there is no Content-Length

src/Tweeper.php: make enclosure validate when there is no Content-Length

When the server does not provide a Content-Length header, curl_getinfo()
would return a negative value for "download_content_length".

However RSS recommends to use 0 when the enclosure's size cannot be
determined.

See: https://www.feedvalidator.org/docs/error/UseZeroForUnknown.html