Fix zts build on PHP5.6 :]
Merge pull request #663 from bjori/php-master-fix

Fix zts build on post PHP5.6 versions :]

* bjori/php-master-fix:
  Fix zts build on PHP5.6 :]
Fixed PHP-928 and PHP-1060: read only properties for inherited classes.
Added two more test cases as per review.
Merged pull request #667
Fixed PHP-1036: Segmentation Fault when querying large collection and the working set is not loaded
Fix similar issues as PHP-1036 by verifying if we actually have a zval before trying to
return it. mongo_rucommand() will throw an exception if its null, so no need to else RETURN_NULL()