fix the "me" field in isMaster responses when using splithorizon

When returning an ismaster response based on the non-default split horizon (see PM-1289), we update the 'primary', 'hosts', 'arbiters', and 'passives' fields of the ismaster response, but not the 'me' field.

This is inconsistently enforced across drivers, and prevents those that do enforce it to be unable to connect to split horizon deployments.

Given the few internal dependencies on the field, it makes the most sense to update "me" as well when generating split horizon responses, as this will allow existing drivers to smoothly access the split horizon functionality.