Properly set selected cells for frozen panes

Properly set the selected cells for worksheets with frozen panes when
writing Xlsx documents. Beforehand, the saved Xlsx documents were
generating corruption warnings when opened in Excel.

Fixes PHPOffice#532
Closes PHPOffice#535

CHANGELOG.md

@@ -24,6 +24,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 - Make newer Excel versions properly recalculate formulas on document open - [#456](https://github.com/PHPOffice/PhpSpreadsheet/issues/456)
 - `Coordinate::extractAllCellReferencesInRange()` throws an exception for an invalid range – [#519](https://github.com/PHPOffice/PhpSpreadsheet/issues/519)
 - Fixed parsing of conditionals in COUNTIF functions - [#526](https://github.com/PHPOffice/PhpSpreadsheet/issues/526)
+- Corruption errors for saved Xlsx docs with frozen panes - [#532](https://github.com/PHPOffice/PhpSpreadsheet/issues/532)
 
 ## [1.2.1] - 2018-04-10