Update birth ref index and death ref index when updating person

[DavidMStraub]

Adding a birth or death event to an existing person in Gramps.js, I noticed that thebirth_ref_index and death_ref_index on the Person object are not set correctly - I had assumed that this was done automatically when committing the change. However, it turned out that in Gramps this needs to be done "manually" by calling the method set_birth_death_index. This PR adds this call to the update_object utility function that is used in the PUT endpoints. A unit test is added for this case.