PHP-1245: Replacing MONGO_METHOD to MongoCollection::insert() calls.

Should we also assert that `z_write_options` is an array or object here, before we try to access its hash table in `php_mongo_collection_insert()`? I realize this is simply a port of the old code, but it looks like an error.

I'll note that the other functions calling `php_mongo_collection_insert()` (e.g. `createIndex`, `ensureIndex`) use `a` as the zpp type.