t5551-http-fetch: Work around broken Accept header in libcurl

t5551-http-fetch: Work around broken Accept header in libcurl

Unfortunately at least one version of libcurl has a bug causing
it to include "Accept: */*" in the same POST request where we have
already asked for "Accept: application/x-git-upload-pack-response".

This is a bug in libcurl, not Git, or our test vector.  The
application has explicitly asked the server for a single content
type, but libcurl has mistakenly also told the server the client
application will accept */*, which is any content type.

Based on the libcurl change log, this "Accept: */*" header bug
may have been fixed in version 7.18.1 released March 30, 2008:

  http://curl.haxx.se/changes.html#7_18_1

Rather than require users to upgrade libcurl we change the test
vector to trim this line out of the 2nd request.

Reported-by: Tarmigan <tarmigan+git@gmail.com>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>