revert: Re-add string type hint to Response::write() parameter

Author: KnifeLemon

flight/net/Response.php

@@ -237,7 +237,7 @@ public function getHeaders(): array
      *
      * @return $this Self reference
      */
-    public function write($str, bool $overwrite = false): self
+    public function write(string $str, bool $overwrite = false): self
     {
         if ($overwrite === true) {
             $this->clearBody();