provide a nicer `to_s` if no preferred display name is defined
display_name spec cleanup

note that our use of `subject{ }` here was incorrect -- the subject
returned was actually the `"display_name"` string from the parent
`describe` block :<
add tests and update the display_name fallback behavior
Merge pull request #2883 from seanlinsley/feature/better-default-display_name

Provide a nicer `to_s` if no preferred display name is defined