BUG/MINOR: httpclient: allow to replace the host header

This patch allows to replace the host header generated by the
httpclient instead of adding a new one, resulting in the server replying
an error 400.

The host header is now generated from the uri only if it wasn't found in
the list of headers.

Also add a new request in the VTC file to test this.

This patch must be backported in 2.5.
diff --git a/reg-tests/lua/lua_httpclient.vtc b/reg-tests/lua/lua_httpclient.vtc
index 9d53cfe..7c55d54 100644
--- a/reg-tests/lua/lua_httpclient.vtc
+++ b/reg-tests/lua/lua_httpclient.vtc
@@ -28,6 +28,14 @@
    expect req.bodylen == 54000
 } -start
 
+server s3 {
+   rxreq
+   txresp -bodylen 54000
+   expect req.method == "GET"
+   expect req.http.host == "foobar.haproxy.local"
+} -start
+
+
 haproxy h1 -conf {
     global
         lua-load ${testdir}/lua_httpclient.lua
@@ -44,7 +52,7 @@
 } -start
 
 client c0 -connect ${h1_fe1_sock} {
-    txreq -url "/" -hdr "vtcport: ${s1_port}" -hdr "vtcport2: ${s2_port}"
+    txreq -url "/" -hdr "vtcport: ${s1_port}" -hdr "vtcport2: ${s2_port}" -hdr "vtcport3: ${s3_port}"
     rxresp
     expect resp.status == 200
 } -run
@@ -52,3 +60,4 @@
 
 server s1 -wait
 server s2 -wait
+server s3 -wait