Fixes a bug when calling `$sheet->freezePane('B2')` without a second argument. The selected cell will now be `B2` instead of the incorrect `B3`. Fixes #389 Closes #393
Use the `PHPUnit\Framework\TestCase` notation instead of `PHPUnit_Framework_TestCase` while extending our TestCases. This will help us migrate to PHPUnit 6, that [no longer support snake case class names](https://github.com/sebastianbergmann/phpunit/blob/master/ChangeLog-6.0.md#changed-1).