Alternative GridFS collection names issue fix.[dev]

There is a problem in source/MongoDb.php, it doesn't work with GridFS collections which is named different to defaults.

This call doesn't pass prefix to the getGridFS and defaults "fs.files" will be used, here it is:
```php
$collection = $self->connection->getGridFS();
```

This PR has a fix for this issue.

But there is one more problem — unable to use different sources for gridfs collection, because of this:
```php
if ($source == "{$_config['gridPrefix']}.files")
```
I'm afraid It's a wrong way to determine the type of collection  — is it gridfs or not.

I'm working now on the solution.