Views collation check should traverse through nested $lookup pipelines

It looks like $lookup's implementation of addInvolvedCollections is incorrect. It neglects to include any namespaces that are referenced in a subpipline, should one exist.

It looks like this method is only used to check that all involved views are using the same collation, so the impact of this bad implementation would be a failure to detect mixed collations within a $lookup subpipeline.