fix: Support Header element within Title elements

We encountered issues with a Word document where this is a valid combination.

src/PhpWord/Element/AbstractContainer.php

@@ -253,7 +253,7 @@ private function checkValidity($method)
             'Footnote' => ['Section', 'TextRun', 'Cell', 'ListItemRun'],
             'Endnote' => ['Section', 'TextRun', 'Cell'],
             'PreserveText' => ['Section', 'Header', 'Footer', 'Cell'],
-            'Title' => ['Section', 'Cell'],
+            'Title' => ['Section', 'Cell', 'Header'],
             'TOC' => ['Section'],
             'PageBreak' => ['Section'],
             'Chart' => ['Section', 'Cell'],