Fixed PHP-883: php_mongo_dbref_create() handling of MongoId values

If a MongoId or scalar is passed as the zid argument, it should be used as-is. Otherwise, assume the argument is a document and access its _id key/property if it is an array/object, respectively.
Fixed PHP-883: createDBRef() collection/DB method should omit $db

This restores original behavior, which was broken by 926d08d6965e88a07e2c9c0cd029484142a79c4e.
Rename ns parameter to collection for clarity

This parameter is the collection name, not the full namespace (i.e. db.collection).
Merged pull request #509