Reader Word2007 : Support Header elements within Title elements (#2674)

* fix: Support Header element within Title elements

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

* docs(changelog): Add note about supporting Header elements within Title

Author: SpraxDev

docs/changes/1.x/1.4.0.md

@@ -17,6 +17,7 @@
 - Allow vAlign and vMerge on Style\Cell to be set to null by [@SpraxDev](https://github.com/SpraxDev) fixing [#2673](https://github.com/PHPOffice/PHPWord/issues/2673) in [#2676](https://github.com/PHPOffice/PHPWord/pull/2676)
 - Reader HTML: Support for differents size units for table by [@Progi1984](https://github.com/Progi1984) fixing [#2384](https://github.com/PHPOffice/PHPWord/issues/2384), [#2701](https://github.com/PHPOffice/PHPWord/issues/2701) in [#2725](https://github.com/PHPOffice/PHPWord/pull/2725)
 - Reader Word2007 : Respect paragraph indent units by [@tugmaks](https://github.com/tugmaks) & [@Progi1984](https://github.com/Progi1984) fixing [#507](https://github.com/PHPOffice/PHPWord/issues/507) in [#2726](https://github.com/PHPOffice/PHPWord/pull/2726)
+- Reader Word2007 : Support Header elements within Title elements by [@SpraxDev](https://github.com/SpraxDev) fixing [#2616](https://github.com/PHPOffice/PHPWord/issues/2616), [#2426](https://github.com/PHPOffice/PHPWord/issues/2426) in [#2674](https://github.com/PHPOffice/PHPWord/pull/2674)
 
 ### Miscellaneous
 

src/PhpWord/Element/AbstractContainer.php

@@ -254,7 +254,7 @@ 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'],