CURLFORM_STREAM: acknowledge CURLFORM_FILENAME

The CURLFORM_STREAM is documented to only insert a file name (and thus
look like a file upload) in the part if CURLFORM_FILENAME is set, but in
reality it always inserted a filename="" and if CURLFORM_FILENAME wasn't
set, it would insert insert rubbish (or possibly crash).

This is now fixed to work as documented, and test 554 has been extended
to verify this.

Reported by: Sascha Swiercy
Bug: http://curl.haxx.se/mail/lib-2011-06/0070.html