fix(auth): don't redeclare BSON variable in plain auth

* Don't redeclare BSON variable
* Update according to PR comments

Author: denizdogan

lib/auth/plain.js

@@ -3,12 +3,12 @@
 var BSON = require('bson');
 
 var f = require('util').format
-  , Binary = BSON.Binary
   , retrieveBSON = require('../connection/utils').retrieveBSON
   , Query = require('../connection/commands').Query
   , MongoError = require('../error');
 
-var BSON = retrieveBSON();
+var BSON = retrieveBSON()
+  , Binary = BSON.Binary;
 
 var AuthSession = function(db, username, password) {
   this.db = db;