docs: fix PHPDoc type mixed in HTTP

system/HTTP/IncomingRequest.php

@@ -671,7 +671,7 @@ class IncomingRequest extends Request
      * with redirect_with_input(). It first checks for the data in the old
      * POST data, then the old GET data and finally check for dot arrays
      *
-     * @return mixed
+     * @return array|string|null
      */
     public function getOldInput(string $key)
     {

system/HTTP/ResponseInterface.php

@@ -202,7 +202,7 @@ interface ResponseInterface
     /**
      * Returns the current body, converted to JSON is it isn't already.
      *
-     * @return mixed|string
+     * @return string|null
      *
      * @throws InvalidArgumentException If the body property is not array.
      */

system/HTTP/ResponseTrait.php

@@ -252,7 +252,7 @@ trait ResponseTrait
     /**
      * Returns the current body, converted to JSON is it isn't already.
      *
-     * @return mixed|string
+     * @return string|null
      *
      * @throws InvalidArgumentException If the body property is not array.
      */