To suport preseve text inside sub container

if we use preseve text inside table, issue fix

https://stackoverflow.com/questions/33070424/phpword-cannot-add-preservetext-in-section

Author: Nishant Bhatt

src/PhpWord/Element/AbstractContainer.php

@@ -254,7 +254,7 @@ private function checkValidity($method)
         // Special condition, e.g. preservetext can only exists in cell when
         // the cell is located in header or footer
         $validSubcontainers = array(
-            'PreserveText'  => array(array('Cell'), array('Header', 'Footer')),
+            'PreserveText'  => array(array('Cell'), array('Header', 'Footer', 'Section')),
             'Footnote'      => array(array('Cell', 'TextRun'), array('Section')),
             'Endnote'       => array(array('Cell', 'TextRun'), array('Section')),
         );