rowspans/colspans not reset when adding a new worksheet using loadIntoExisting

[danielcherrington]

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)

What is the expected behavior?

rowspans/colspans are respected for each HTML document added to an existing spreadsheet as a new worksheet. The protected $rowspan class property should be emptied on each call to "loadIntoExisting"

What is the current behavior?

rowspans/colspans are cached by column reference across the entire spreadsheet meaning additional sheets added to the spreadsheet start at the wrong column index

What are the steps to reproduce?

You will need two html files containing tables with rowspans/colspans defined

Please provide a Minimal, Complete, and Verifiable example of code that exhibits the issue without relying on an external Excel file or a web server:

<?php

require __DIR__ . '/vendor/autoload.php';

use PhpOffice\PhpSpreadsheet\Spreadsheet;
use PhpOffice\PhpSpreadsheet\Writer\Xlsx;

$inputFileType = 'Html';

$inputFileNames = [
    "1.html",
    "2.html"
];

$reader = \PhpOffice\PhpSpreadsheet\IOFactory::createReader($inputFileType);
$inputFileName = array_shift($inputFileNames);
$spreadsheet = $reader->load($inputFileName);
$spreadsheet->getActiveSheet()
    ->setTitle(pathinfo($inputFileName, PATHINFO_BASENAME));

foreach ($inputFileNames as $sheet => $inputFileName) {
    $reader->setSheetIndex($sheet+1);
    $reader->loadIntoExisting($inputFileName, $spreadsheet);
    $spreadsheet->getActiveSheet()
        ->setTitle(pathinfo($inputFileName, PATHINFO_BASENAME));
}

$writer = \PhpOffice\PhpSpreadsheet\IOFactory::createWriter($spreadsheet, "Xlsx");
$writer->save("merged.xlsx");

Which versions of PhpSpreadsheet and PHP are affected?

Latest (1.3.1)