In the following test cases replset45 is a random string and there is no replica set members to connect to. 

Case 1 and 2 should be equivalent. 

1. mongo "mongodb://localhost:27017,localhost:27018,localhost:27019/?replicaSet=replset45"
Correct behavior
3.6.10 - Cannot reach any nodes for set replset45
4.0.5 - Cannot reach any nodes for set replset45

2. mongo "mongodb://localhost:27017,localhost:27018,localhost:27019/?replicaset=replset45"
3.6.10 - Connection success
4.0.5 - Connection success