retry_on says that it can take an Array of hosts, but it will fail if one is passed

[trevor-vaughan]

retry_on() indicates that it can take an Array of hosts but its call to log_prefix will cause a failure during execution since it is working on an Array.

I would propose serializing the execution of retry_on() so that it does support an Array of hosts but I can also see simply rejecting anything that is not a single host (which is the way that it is currently written).