fix HTTP-version case-sensitivity on Response.php

Based on rfc7230 : HTTP-version is case-sensitive.
For example, wrk don't work with `Http/1.1` version but work with `HTTP/1.1`.

src/Response/Response.php

@@ -154,7 +154,7 @@ public function rawBody(): string
 
     public function __toString()
     {
-        $head = "Http/1.1 $this->status {$this->phrases[$this->status]}\r\n";
+        $head = "HTTP/1.1 $this->status {$this->phrases[$this->status]}\r\n";
 
         foreach ($this->headers as $name => $value) {
             if (\is_array($value)) {