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/test1434

@@ -81,8 +81,8 @@ http://%HOSTIP:%HTTPPORT/1434 -C 100
 </strip>
 <protocol>
 GET /1434 HTTP/1.1
-Range: bytes=100-
 Host: %HOSTIP:%HTTPPORT
+Range: bytes=100-
 Accept: */*
 
 </protocol>