Zend\Filter\Compress\Bz2 and Gz should be fully PHP >= 5.4 compatible

[wdJer]

Currently it's not possible to decompress, without any Warning, with Zend\Filter\Compress\Bz2 or Zend\Filter\Compress\Gz and direct string input in PHP >= 5.4
In the tests of both classes the testcase testBasicUsage has a incomplete marker for PHP >= 5.4

PHP WARNING: file_exists() expects parameter 1 to be a valid path, string given