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

tests/data/test3

@@ -48,8 +48,8 @@ HTTP POST with auth and contents but with content-length set to 0
 </strip>
 <protocol nonewline="yes">
 POST /3 HTTP/1.1
-Authorization: Basic ZmFrZTotdXNlcg==
 Host: %HOSTIP:%HTTPPORT
+Authorization: Basic ZmFrZTotdXNlcg==
 Accept: */*
 Content-Length: 37
 Content-Type: application/x-www-form-urlencoded