CURLFORM_BUFFER: insert filename as documented

A regression where CURLFORM_BUFFER stopped to properly insert the file
name part in the formpart. Bug introduced in commit f851f768.

Added CURLFORM_BUFFER use to test 554 to verify this.

Bug: http://curl.haxx.se/mail/lib-2011-07/0176.html
Reported by: Henry Ludemann