t5551: Remove header from curl cookie file

The URL included in the header appears to vary from curl version to
curl version.  Since we only care about the final few lines, only test
them.  However, make sure the blank line after the header is still
included to make sure there are no extra cookie lines.

Signed-off-by: Brian Gernhardt <brian@gernhardtsoftware.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>