Rowspans/colspans were incorrect when adding worksheet using loadIntoExisting

Rowspans/colspans are now 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`.

Fixes #619
Fixes #620

CHANGELOG.md

@@ -67,6 +67,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 - Chained operations on cell ranges involving borders operated on last cell only [#428](https://github.com/PHPOffice/PhpSpreadsheet/issues/428)
 - Avoid memory exhaustion when cloning worksheet with a drawing [#437](https://github.com/PHPOffice/PhpSpreadsheet/issues/437)
 - Migration tool keep variables containing $PHPExcel untouched [#598](https://github.com/PHPOffice/PhpSpreadsheet/issues/598)
+- Rowspans/colspans were incorrect when adding worksheet using loadIntoExisting [#619](https://github.com/PHPOffice/PhpSpreadsheet/issues/619)
 
 ## [1.3.1] - 2018-06-12
 

src/PhpSpreadsheet/Reader/Html.php

@@ -554,6 +554,7 @@ class Html extends BaseReader
         $row = 0;
         $column = 'A';
         $content = '';
+        $this->rowspan = [];
         $this->processDomElement($dom, $spreadsheet->getActiveSheet(), $row, $column, $content);
 
         // Return