Invalid date calendar format detection while loading several spreadsheets

[K0nias]

This is:

- [x] a bug report
- [ ] a feature request
- [ ] **not** a usage question (ask them on https://stackoverflow.com/questions/tagged/phpspreadsheet or https://gitter.im/PHPOffice/PhpSpreadsheet)

I came across this issue while I was writing tests with loading several spreadsheets and some datetime values was clearly invalid.

What is the expected behavior?

Loading numerious xlsx files should not share global state. Problematic is \PhpOffice\PhpSpreadsheet\Shared\Date::$excelCalendar

Problematic code https://github.com/PHPOffice/PhpSpreadsheet/blob/master/src/PhpSpreadsheet/Reader/Xlsx.php#L662

What is the current behavior?

\PhpOffice\PhpSpreadsheet\Shared\Date::$excelCalendar configuration can be shared across multiple spreadsheets

What are the steps to reproduce?

<?php

include 'vendor/autoload.php';

$reader = new \PhpOffice\PhpSpreadsheet\Reader\Xlsx();

// this method call config global value of \PhpOffice\PhpSpreadsheet\Shared\Date::setExcelCalendar
$reader->load(/* load file with set $xmlWorkbook->workbookPr and with workbookPr['date1904'] */);

// second load of another spreadsheet can be problematic if spreadsheet doesnt have "workbookPr" element. Then
// previously configurated global value of \PhpOffice\PhpSpreadsheet\Shared\Date::$excelCalendar is used

$reader->load(/* load file with missing $xmlWorkbook->workbookPr */);

working example with testing data can be found on gist

Which versions of PhpSpreadsheet and PHP are affected?

1.7.0

but referenced code is 3-4 years old so it could be many versions

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs.
If this is still an issue for you, please try to help by debugging it further and sharing your results.
Thank you for your contributions.

[oleibman]

Reopening, to be closed properly by a PR which will fix the problem.