[HttpFoundation] Fixed phpdoc for get method of HeaderBag

Author: JanHort

HeaderBag.php

@@ -101,11 +101,11 @@ public function add(array $headers)
     /**
      * Returns a header value by name.
      *
-     * @param string          $key     The header name
-     * @param string|string[] $default The default value
-     * @param bool            $first   Whether to return the first value or all header values
+     * @param string               $key     The header name
+     * @param string|string[]|null $default The default value
+     * @param bool                 $first   Whether to return the first value or all header values
      *
-     * @return string|string[] The first header value or default value if $first is true, an array of values otherwise
+     * @return string|string[]|null The first header value or default value if $first is true, an array of values otherwise
      */
     public function get($key, $default = null, $first = true)
     {