Ensure Client's $writeConcern property is initialized
Ensure Client's $writeConcern property is initialized

Fixes a bug introduced in 21b7d92aef4def00abade10911bae41f1b5ea378 for PHPLIB-200

jmikola/mongo-php-library,derickr/mongo-php-library,jmikola/mongo-php-library,mongodb/mongo-php-library,mongodb/mongo-php-library,10gen-labs/mongo-php-library-prototype,derickr/mongo-php-library-prototype,mongodb/mongo-php-library,jmikola/mongo-php-library-prototype,mongodb-labs/mongo-php-library-prototype,mongodb/mongo-php-library,jmikola/mongo-php-library,jmikola/mongo-php-library,derickr/mongo-php-library