Use lwan_request_get_host() instead of the generic get_header()

The Host header is ready to be fetched from the request helper struct,
so fetch it rather than looping through all the headers to find the
one we're looking for!

src/samples/pastebin/main.c

@@ -121,7 +121,7 @@ static enum lwan_http_status post_paste(struct lwan_request *request,
             cache_coro_get_and_ref_entry(pastes, request->conn->coro, key);
 
         if (paste) {
-            const char *host_hdr = lwan_request_get_header(request, "Host");
+            const char *host_hdr = lwan_request_get_host(request);
 
             if (!host_hdr)
                 return HTTP_BAD_REQUEST;
@@ -140,7 +140,7 @@ static enum lwan_http_status post_paste(struct lwan_request *request,
 static enum lwan_http_status doc(struct lwan_request *request,
                                  struct lwan_response *response)
 {
-    const char *host_hdr = lwan_request_get_header(request, "Host");
+    const char *host_hdr = lwan_request_get_host(request);
 
     if (!host_hdr)
         return HTTP_BAD_REQUEST;