[Zend\Filter\Compress] added PHP 5.4 support for strings in Bz2 and Gz decompress

With this PR Zend\Filter\Compress\Bz2 and Zend\Filter\Compress\Gz are capable of decompressing Strings in PHP 5.4 without throwing a Warning like stated in #5788
In my research I found out, that the only problem are null byte characters, so we go for a check if there is a null byte char in the given content.
The skipped tests are working now in PHP >= 5.4