Use soup_message_headers_replace(), not soup_message_headers_append(),

Use soup_message_headers_replace(), not soup_message_headers_append(),

	* libsoup/soup-message.c (soup_message_set_auth)
	(soup_message_set_proxy_auth): Use soup_message_headers_replace(),
	not soup_message_headers_append(), since only a single
	Authorization/Proxy-Authorization header is allowed. #512517.

	* libsoup/soup-auth-manager-ntlm.c (ntlm_request_started): Don't
	set an NTLM Authorization header if the message already has a
	Basic or Digest one.

	* tests/ntlm-test.c: Add some Basic auth and mixed NTLM/Basic auth
	tests

svn path=/trunk/; revision=1059