http: always send Host: header as first header

...after the method line:

 "Since the Host field-value is critical information for handling a
 request, a user agent SHOULD generate Host as the first header field
 following the request-line." / RFC 7230 section 5.4

Additionally, this will also make libcurl ignore multiple specified
custom Host: headers and only use the first one. Test 1121 has been
updated accordingly

Bug: http://curl.haxx.se/bug/view.cgi?id=1491
Reported-by: Rainer Canavan