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 f851f76857.

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

tests/data/test554

@@ -45,7 +45,7 @@ s/boundary=----------------------------[a-z0-9]*/boundary=----------------------
 POST /554 HTTP/1.1
 Host: %HOSTIP:%HTTPPORT
 Accept: */*
-Content-Length: 561
+Content-Length: 732
 Expect: 100-continue
 Content-Type: multipart/form-data; boundary=----------------------------
 
@@ -67,6 +67,11 @@ postit2.c
 Content-Disposition: form-data; name="submit"
 
 send
+------------------------------
+Content-Disposition: form-data; name="somename"; filename="somefile.txt"
+Content-Type: application/octet-stream
+
+blah blah
 --------------------------------
 </protocol>
 </verify>