HTTP: empty chunked POST ended up in two zero size chunks

When doing a chunked-encoded POST with -d (CURLOPT_POSTFIELDS) and the
size of the POST was zero length, it made libcurl first send a zero
chunk and then the terminating one. This could confuse a receiver and it
should rather just send the terminating chunk as it does with this fix.

Test case 1333 is added to verify.

Bug: http://curl.haxx.se/mail/archive-2012-04/0060.html
Reported by: Arnaud Compan