[ServerRequest] Host header is not set when URI has one

> You can opt-in to preserving the original state of the Host header by passing true for the second ($preserveHost) argument. When this argument is set to true, the returned request will not update the Host header of the returned message -- unless the message contains no Host header.

**unless the message contains no Host header.** So $preserveHost MUST be ignored if message does not have a Host header

src/RequestTrait.php

@@ -26,6 +26,7 @@
  * @property array $headers
  * @property array $headerNames
  * @property StreamInterface $stream
+ * @method bool hasHeader(string $header)
  */
 trait RequestTrait
 {
@@ -251,7 +252,7 @@ public function withUri(UriInterface $uri, $preserveHost = false)
         $new = clone $this;
         $new->uri = $uri;
 
-        if ($preserveHost) {
+        if ($preserveHost && $this->hasHeader('Host')) {
             return $new;
         }