Fix for BC break caused by PR #87

[PR 87](https://github.com/doctrine/DoctrineMongoODMModule/pull/87) causes a BC break.

The `ConnectionFactory`'s `setConnectionString(...)` method casts `$connectionString` to `string`. This means that [this condition](https://github.com/doctrine/DoctrineMongoODMModule/blob/2a3acdf7f87d18617b9f23b16600ae255a5dee70/src/DoctrineMongoODMModule/Service/ConnectionFactory.php#L48) can never evaluate to true, which makes it impossible to use anything but `connectionString` in the configuration.