Aggregation helper fails on simplified string form for $merge stage

Originally reported in spring-data-mongodb#3743.

In AggregateIterableImpl.getOutNamespace method
BsonDocument mergeDocument = lastStageDocument.getDocument("$merge")
this line does not consider simple String type when $merge use the simplified form, like {$merge: <collection>}

The workaround is to use the long form of $merge, as documented here.