PHP-1011: Fix MongoDB inheritance of write concern strings

The default values for default_w and default_wstring are -1 and NULL, respectively. There should never be a point where both options have non-default values.

Additionally, avoid a memory leak by freeing default_wstring if we would replace a previously set value for the "w" option.
PHP-1012: Fix MongoCollection is_gle_op() logic

Previously, is_gle_op() would ignore the write concern set on the MongoCollection instance, which is inherited through MongoDB. The local write concern should be consulted over the MongoClient options; however, MongoClient's fsync/journal options should still be checked, since those are not set on MongoDB and MongoCollection instances (only through method options).

This also fixes tests for PHP-569, which were originally reported as invalid in PHP-900.
Merge pull request #613