MAPG-199 fix issues found by PHPStan

src/Http/Response.php

@@ -14,12 +14,12 @@ class Response implements IResponse
         $this->headers = $headers;
     }
 
-    public function getBody()
+    public function getBody(): string
     {
         return $this->body;
     }
 
-    public function getHeaders()
+    public function getHeaders(): array
     {
         return $this->headers;
     }