The common use case for this function is to add a default tag if the
reference only has a name. The current behavior only adds the default
tag if there is no *tag*, which requires most callers to check for a
digest. Change the behavior to only add default tags to name-only
references, and change the name to reflect this. The documentation
already described the new behavior, so it does not need to be changed.

Signed-off-by: Aaron Lehmann <aaron.lehmann@docker.com>