Error message when calling db.collection in strict mode without a callback isn't descriptive


I'm doing this while trying to figure out mongo:

```javascript
var collection = db.collection('someStuff', {strict:true});
if ( !collection ) {
	db.createCollection('someStuff', function ( e, r ) {
		db.close();
	});
};
```

I realize (now) that I'm not supposed to be assigning the return in strict mode. I'm flagging the way that the error is reported.

```
C:\TryMongo\node_modules\mongodb\lib\mongodb\connection\base.js:246
        throw message;
              ^
TypeError: undefined is not a function
    at C:\TryMongo\node_modules\mongodb\lib\mongodb\db.js:548:40
    at C:\TryMongo\node_modules\mongodb\lib\mongodb\db.js:456:7
    at C:\TryMongo\node_modules\mongodb\lib\mongodb\cursor.js:172:16
    at commandHandler (C:\TryMongo\node_modules\mongodb\lib\mongodb\cursor.js:720:16)
    at C:\TryMongo\node_modules\mongodb\lib\mongodb\db.js:1923:9
    at Server.Base._callHandler (C:\TryMongo\node_modules\mongodb\lib\mongodb\connection\base.js:448:41)
    at C:\TryMongo\node_modules\mongodb\lib\mongodb\connection\server.js:481:18
    at MongoReply.parseBody (C:\TryMongo\node_modules\mongodb\lib\mongodb\responses\mongo_reply.js:68:5)
    at null.<anonymous> (C:\TryMongo\node_modules\mongodb\lib\mongodb\connection\server.js:439:20)
    at emit (events.js:95:17)
```

The issue is that no callback is passed to Db.prototype.collection.
So the thrown error, in self.listCollection(,  is 'undefined is not a function' instead of something like 'callbacks are required in strict mode'

I've simply added a throw with a descriptive message if !callback.

Was:
```javascript
  // Strict mode
  self.listCollections(collectionName, function(err, collections) {
```
Is
```javascript
  // Strict mode
  if ( !callback )
      throw utils.toError(f("A callback is required in strict mode. While getting collection %s.", collectionName));

  self.listCollections(collectionName, function(err, collections) {
```