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/test1041

@@ -57,8 +57,8 @@ http://%HOSTIP:%HTTPPORT/1041 -Tlog/test1041.txt -C -
 # it can skip part of the file or not.  Instead, it sends the entire file.
 <protocol>
 PUT /1041 HTTP/1.1
-Content-Range: bytes 0-99/100
 Host: %HOSTIP:%HTTPPORT
+Content-Range: bytes 0-99/100
 Accept: */*
 Content-Length: 100
 Expect: 100-continue