changed case: use new host name for subsequent HTTP requests

When a HTTP connection is re-used for a subsequent request without
proxy, it would always re-use the Host: header of the first request. As
host names are case insensitive it would make curl send another host
name case that what the particular request used.

Now it will instead always use the most recent host name to always use
the desired casing.

Added test case 1318 to verify.

Bug: http://curl.haxx.se/mail/lib-2011-12/0314.html
Reported by: Alex Vinnik